diff --git a/source/Makefile b/source/Makefile
new file mode 100644
index 0000000..036a68f
--- /dev/null
+++ b/source/Makefile
@@ -0,0 +1,70 @@
+# Makefile for ComfyUI-AudioBatch project
+
+# Use the python from the current virtual environment
+# This assumes you have activated your ComfyUI venv before running `make`
+# If not, you might need to specify the path to the python executable
+PYTHON := python
+
+# Define test command with necessary pytest options
+# --import-mode=append: Prevents pytest from prepending the test root to sys.path,
+#                       solving many relative/absolute import conflicts.
+# -v: Verbose output.
+# -s: Show print statements (useful for debugging tests).
+# --ignore=path/to/ignore: Example of ignoring a directory if needed.
+PYTEST_CMD := pytest --import-mode=append -v -s
+
+# --- Targets ---
+
+.PHONY: help test test-one lint format check
+
+help:
+	@echo "Makefile for ComfyUI-AudioBatch"
+	@echo ""
+	@echo "Usage:"
+	@echo "  make help           Show this help message."
+	@echo "  make install        Install development dependencies from requirements-dev.txt."
+	@echo "  make test           Run all tests in the 'tests/' directory."
+	@echo "  make test-one       Run a specific test file. Usage: make test-one file=tests/test_audio_blend.py"
+	@echo "  make lint           Run flake8 linter on the source code."
+	@echo "  make format         Run black code formatter on the source code."
+	@echo "  make check          Run all checks (lint, type check, tests)."
+	@echo "  make type-check     Run mypy for static type checking."
+
+
+# Target to install development dependencies
+install:
+	@echo "Installing development dependencies..."
+	@$(PYTHON) -m pip install -r requirements-dev.txt
+
+# Target to run all tests
+test:
+	@echo "Running all tests..."
+	@$(PYTEST_CMD) tests/
+
+# Target to run a single test file, specified with `make test-one file=...`
+test-one:
+	@if [ -z "$(file)" ]; then \
+		echo "Error: Please specify a file to test. Usage: make test-one file=tests/your_test_file.py"; \
+		exit 1; \
+	fi
+	@echo "Running test for file: $(file)"
+	@$(PYTEST_CMD) $(file)
+
+# Target to run the linter (e.g., flake8)
+lint:
+	@echo "Running linter (flake8)..."
+	flake8 .
+
+# Target to run mypy for type checking
+type-check:
+	@echo "Running static type checker (mypy)..."
+	@$(PYTHON) -m mypy .
+
+# Target to format the code (e.g., black)
+format:
+	@echo "Formatting code (black)..."
+	@$(PYTHON) -m black .
+
+# Target to run all checks together
+check: lint type-check test
+	@echo "All checks completed."
\ No newline at end of file
diff --git a/source/tests/bootstrap/__init__.py b/source/tests/bootstrap/__init__.py
new file mode 100644
index 0000000..fae4571
--- /dev/null
+++ b/source/tests/bootstrap/__init__.py
@@ -0,0 +1,55 @@
+# Copyright (c) 2025 Salvador E. Tropea
+# Copyright (c) 2025 Instituto Nacional de Tecnología Industrial
+# License: GPLv3
+# Project: ComfyUI-AudioSeparation
+#
+# Why such a complex thing?
+# Python imports are broken by design, and here we hit a huge limitation:
+# 1. ComfyUI nodes MUST use relative imports, if you don't do it things like "import utils" becomes ambiguous
+#    You might think this can be overcome polluting the sys.path, but this isn't true. If ComfyUI, or some
+#    other node, already imported an module named "utils" you'll get the already imported module, not the one
+#    you want. And polluting sys.path can make other nodes, or ComfyUI itself, import the wrong module when
+#    they use a "non top-level import".
+#    Conclusion: The only safe way to import from a ComfyUI node is by using relative imports.
+# 2. We have tools in the "tool" subdir, they are intended to run as standalone scripts. They can't use
+#    relative imports to access the node sub modules because you'll hit the error "ImportError: attempted
+#    relative import with no known parent package". So imports in tools MUST be absolute. This can be solved
+#    adding the node root to sys.path. Here we are not polluting the sys.path because we are the top-level.
+#    Conclusion: This script is used to solve adding the correct path to sys.path
+# 3. As you MUST use relative imports in the node sub-modules, when a sub-module depends on another sub-module
+#    it will do something like "from ..XXXX", this is OK when all is relative. But tools are using absolute
+#    imports, so you'll hit the error "ImportError: attempted relative import beyond top-level package"
+#    Conclusion: All sub-modules must be wrapped by an umbrella sub-module, this is what "source" is.
+#
+# Structure:
+# Node-name/       <-- is a module from ComfyUI point of view, dynamically imported using importlib
+# \-- __init__.py  <-- needed because we are a module, relative imports
+# \-- nodes.py     <-- relative imports
+# |
+# \-- source/      <-- umbrella package, just to solve relative imports
+# |   \-- __init__.py
+# |   |
+# |   \-- utils/   <-- a submodule, uses relative imports
+# |   |   \-- __init__.py
+# |   |   \-- misc.py
+# |   |
+# |   \-- db/               <-- another submodule, uses relative imports
+# |       \-- __init__.py
+# |       \-- models_db.py  <-- Must use relative imports, can access ..utils.misc
+# |
+# \-- tool/
+#     \-- batch_convert.py  <-- a tool, must use absolute import of "source"
+#     |
+#     \-- bootstrap/
+#         \-- __init__.py   <-- THIS file, adds to sys.path to make "source" available
+
+# --- BOOTSTRAP: Make the script aware of the project root ---
+import os
+import sys
+# 1. Get the absolute path of the current script's directory (tool/)
+script_dir = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+# 2. Get the project root by going one directory up (MyAwesomeNode/)
+project_root = os.path.dirname(script_dir)
+# 3. Add the project root to the Python path
+if project_root not in sys.path:
+    sys.path.insert(0, project_root)
diff --git a/source/tests/test_audio_batch.py b/source/tests/test_audio_batch.py
new file mode 100644
index 0000000..2a2dc8d
--- /dev/null
+++ b/source/tests/test_audio_batch.py
@@ -0,0 +1,142 @@
+"""
+Regression tests for the AudioBatch node in ComfyUI-AudioBatch.
+"""
+
+import bootstrap  # noqa: F401
+import torch
+import pytest
+from nodes.nodes_audio import AudioBatch
+
+
+# Helper function (can be moved to a shared conftest.py or test_utils.py later)
+def create_dummy_audio(batch_size, channels, samples, sr, device='cpu'):
+    waveform = torch.randn(batch_size, channels, samples, device=device, dtype=torch.float32)
+    return {"waveform": waveform, "sample_rate": sr}
+
+
+@pytest.fixture
+def audio_batch_node():
+    return AudioBatch()
+
+
+def test_batch_simple_match(audio_batch_node):
+    """Tests batching two identical audio inputs."""
+    sr = 44100
+    audio1 = create_dummy_audio(1, 2, 1000, sr)
+    audio2 = create_dummy_audio(1, 2, 1000, sr)
+
+    (result_audio,) = audio_batch_node.batch_audio(audio1, audio2)
+
+    expected_waveform = torch.cat((audio1['waveform'], audio2['waveform']), dim=0)
+
+    assert result_audio['sample_rate'] == sr
+    assert result_audio['waveform'].shape == (2, 2, 1000)
+    assert torch.allclose(result_audio['waveform'], expected_waveform)
+
+
+def test_batch_different_batch_sizes(audio_batch_node):
+    """Tests batching inputs with B=2 and B=3, expecting an output of B=5."""
+    sr = 44100
+    audio1 = create_dummy_audio(2, 2, 1000, sr)
+    audio2 = create_dummy_audio(3, 2, 1000, sr)
+
+    (result_audio,) = audio_batch_node.batch_audio(audio1, audio2)
+
+    assert result_audio['waveform'].shape[0] == 5  # 2 + 3
+
+
+def test_batch_different_lengths_padding(audio_batch_node):
+    """Tests if the shorter audio is padded to match the longer one."""
+    sr = 44100
+    audio1 = create_dummy_audio(1, 2, 500, sr)
+    audio2 = create_dummy_audio(1, 2, 1000, sr)
+
+    (result_audio,) = audio_batch_node.batch_audio(audio1, audio2)
+
+    # Both items in the output batch should have the length of the longer audio
+    assert result_audio['waveform'].shape[2] == 1000
+    # The first item should have been padded
+    assert result_audio['waveform'][0].shape[1] == 1000
+
+
+def test_batch_mono_and_stereo(audio_batch_node):
+    """Tests if the mono audio is converted to stereo to match the other input."""
+    sr = 44100
+    audio1_mono = create_dummy_audio(1, 1, 1000, sr)
+    audio2_stereo = create_dummy_audio(1, 2, 1000, sr)
+
+    (result_audio,) = audio_batch_node.batch_audio(audio1_mono, audio2_stereo)
+
+    # Both items in the output batch should be stereo
+    assert result_audio['waveform'].shape[1] == 2
+    assert result_audio['waveform'].shape[0] == 2
+
+
+def test_batch_different_sample_rates(audio_batch_node):
+    """Tests if audio2 is resampled to match audio1's sample rate."""
+    sr1, sr2 = 44100, 22050
+    samples1, samples2 = 2000, 1000
+    audio1 = create_dummy_audio(1, 2, samples1, sr1)
+    audio2 = create_dummy_audio(1, 2, samples2, sr2)
+
+    (result_audio,) = audio_batch_node.batch_audio(audio1, audio2)
+
+    # Output SR should be sr1
+    assert result_audio['sample_rate'] == sr1
+    # Length of audio2 after resampling is approx samples2 * (sr1/sr2) = 1000 * 2 = 2000
+    # So both items should have length 2000
+    assert result_audio['waveform'].shape[2] == 2000
+
+
+def test_batch_full_alignment_and_content(audio_batch_node):
+    """
+    Strictly tests the end-to-end alignment (SR, channels, length) and content
+    of the batching process.
+    """
+    # --- Input 1: A stereo signal at 48000 Hz, 1 second long ---
+    sr1 = 48000
+    len1 = sr1 * 1
+    # Create L/R channels that are easy to identify (ramps)
+    wf1_l = torch.linspace(0.1, 0.2, len1)
+    wf1_r = torch.linspace(0.3, 0.4, len1)
+    wf1_orig = torch.stack((wf1_l, wf1_r), dim=0).unsqueeze(0)  # (1, 2, 48000)
+    audio1 = {"waveform": wf1_orig, "sample_rate": sr1}
+
+    # --- Input 2: A mono signal at 16000 Hz, 1.5 seconds long ---
+    sr2 = 16000
+    len2 = int(sr2 * 1.5)
+    wf2_orig = (torch.ones(1, 1, len2) * 0.5)  # Constant value mono signal, (1, 1, 24000)
+    audio2 = {"waveform": wf2_orig, "sample_rate": sr2}
+
+    # --- Action ---
+    (result_audio,) = audio_batch_node.batch_audio(audio1, audio2)
+    result_wf = result_audio['waveform']
+
+    # --- Assertions ---
+    # 1. Final parameters should match audio1's SR, be stereo, and have the longest length after resampling.
+    target_sr = sr1  # 48000
+    target_channels = 2
+    len2_resampled = int(len2 * (target_sr / sr2))  # 24000 * 3 = 72000
+    target_samples = max(len1, len2_resampled)  # max(48000, 72000) = 72000
+
+    assert result_audio['sample_rate'] == target_sr
+    assert result_wf.shape == (2, target_channels, target_samples)  # B=2, C=2, N=72000
+
+    # 2. Verify content of the first item (was audio1)
+    item1_output = result_wf[0]  # (2, 72000)
+    # The first part should match the original audio1
+    assert torch.allclose(item1_output[:, :len1], wf1_orig.squeeze(0))
+    # The padded part should be zeros
+    assert torch.all(item1_output[:, len1:] == 0)
+
+    # 3. Verify content of the second item (was audio2)
+    item2_output = result_wf[1]  # (2, 72000)
+    # It was mono, so both output channels should be identical
+    assert torch.allclose(item2_output[0, :], item2_output[1, :])
+    # The content should be a resampled version of the constant 0.5 signal.
+    # A resampled constant signal should still be (roughly) constant.
+    # We check the first part, up to where the original content was.
+    resampled_part = item2_output[0, :len2_resampled]
+    # The average value should be very close to 0.5. Due to filtering (Gibbs effect),
+    # it won't be perfect, especially at the edges.
+    assert torch.mean(resampled_part[100:-100]).item() == pytest.approx(0.5, abs=1e-3)
diff --git a/source/tests/test_audio_blend.py b/source/tests/test_audio_blend.py
new file mode 100644
index 0000000..2953d78
--- /dev/null
+++ b/source/tests/test_audio_blend.py
@@ -0,0 +1,173 @@
+"""
+Regression tests for the AudioBlend node in ComfyUI-AudioBatch.
+"""
+
+import torch
+import pytest
+import logging
+
+import bootstrap  # noqa: F401
+# Now we can import the classes and functions to be tested
+# We need AudioBlend, but it uses AudioBatchAligner internally, which is in the same file.
+# We also import the logger to see node output during tests.
+from nodes.nodes_audio import AudioBlend, logger as node_logger
+
+# Configure logging for tests to see output from the node
+node_logger.setLevel(logging.DEBUG)
+
+
+# --- Pytest Fixtures (Reusable Setup) ---
+
+@pytest.fixture
+def audio_blend_node():
+    """Provides an instance of the AudioBlend node for tests."""
+    return AudioBlend()
+
+
+# --- Helper Functions ---
+
+def create_dummy_audio(batch_size: int, channels: int, samples: int, sr: int,
+                       device: str = 'cpu') -> dict:
+    """Creates a standard ComfyUI AUDIO dictionary for testing."""
+    # Using torch.ones for predictable values, multiplied by a small float
+    # to avoid pure 1s which can mask errors in blending.
+    waveform = torch.ones(batch_size, channels, samples, device=device, dtype=torch.float32) * 0.5
+    return {"waveform": waveform, "sample_rate": sr}
+
+
+# --- Test Cases for AudioBlend ---
+
+def test_blend_simple_match(audio_blend_node):
+    """
+    Tests blending two identical audio inputs.
+    This is the "happy path" where no resampling, channel conversion, or padding is needed.
+    """
+    sr = 44100
+    audio1 = create_dummy_audio(batch_size=1, channels=2, samples=1000, sr=sr)
+    audio2 = create_dummy_audio(batch_size=1, channels=2, samples=1000, sr=sr)
+    gain1, gain2 = 0.5, 0.8
+
+    (result_audio,) = audio_blend_node.blend_audio(audio1, gain1, gain2, audio2)
+
+    # Assertions
+    expected_waveform = (audio1['waveform'] * gain1) + (audio2['waveform'] * gain2)
+    assert result_audio['sample_rate'] == sr
+    assert result_audio['waveform'].shape == expected_waveform.shape
+    assert torch.allclose(result_audio['waveform'], expected_waveform)
+
+
+def test_blend_no_audio2(audio_blend_node):
+    """
+    Tests the case where the optional `audio2` input is not provided (is None).
+    """
+    sr = 44100
+    audio1 = create_dummy_audio(batch_size=2, channels=1, samples=1000, sr=sr)
+    gain1, gain2 = 0.7, 1.0  # gain2 should be ignored
+
+    (result_audio,) = audio_blend_node.blend_audio(audio1, gain1, gain2, audio2=None)
+
+    # Assertions
+    expected_waveform = audio1['waveform'] * gain1
+    assert result_audio['sample_rate'] == sr
+    assert result_audio['waveform'].shape == expected_waveform.shape
+    assert torch.allclose(result_audio['waveform'], expected_waveform)
+
+
+def test_blend_mismatched_sample_rate(audio_blend_node):
+    """
+    Tests if audio2 is correctly resampled to match audio1's sample rate.
+    """
+    sr1, sr2 = 44100, 22050
+    audio1 = create_dummy_audio(batch_size=1, channels=2, samples=2000, sr=sr1)
+    audio2 = create_dummy_audio(batch_size=1, channels=2, samples=1000, sr=sr2)
+    gain1, gain2 = 1.0, 1.0
+
+    (result_audio,) = audio_blend_node.blend_audio(audio1, gain1, gain2, audio2)
+
+    # Assertions
+    # Output SR should match audio1
+    assert result_audio['sample_rate'] == sr1
+    # Output length should match audio1, as it's longer after resampling audio2
+    assert result_audio['waveform'].shape[2] == audio1['waveform'].shape[2]
+    # Output batch and channels should match
+    assert result_audio['waveform'].shape[0] == 1
+    assert result_audio['waveform'].shape[1] == 2
+
+
+def test_blend_mono_and_stereo(audio_blend_node):
+    """
+    Tests if a mono input is correctly converted to stereo when blended with a stereo input.
+    """
+    sr = 44100
+    audio1_mono = create_dummy_audio(batch_size=1, channels=1, samples=1000, sr=sr)
+    audio2_stereo = create_dummy_audio(batch_size=1, channels=2, samples=1000, sr=sr)
+    gain1, gain2 = 1.0, 1.0
+
+    (result_audio,) = audio_blend_node.blend_audio(audio1_mono, gain1, gain2, audio2_stereo)
+
+    # Assertions
+    assert result_audio['sample_rate'] == sr
+    # Output should be stereo (2 channels)
+    assert result_audio['waveform'].shape[1] == 2
+    assert result_audio['waveform'].shape[2] == 1000
+
+    # Check if the mono part was correctly duplicated and blended.
+    # The two output channels should differ only by the difference in audio2's stereo channels.
+    output_diff = result_audio['waveform'][:, 0, :] - result_audio['waveform'][:, 1, :]
+    input2_diff = audio2_stereo['waveform'][:, 0, :] - audio2_stereo['waveform'][:, 1, :]
+    # Since gain2 is 1.0, the difference should be the same.
+    assert torch.allclose(output_diff, input2_diff)
+
+
+def test_blend_mismatched_length(audio_blend_node):
+    """
+    Tests if the shorter audio is correctly padded to match the longer one.
+    """
+    sr = 44100
+    len_short, len_long = 500, 1000
+    audio1_short = create_dummy_audio(batch_size=1, channels=2, samples=len_short, sr=sr)
+    audio2_long = create_dummy_audio(batch_size=1, channels=2, samples=len_long, sr=sr)
+    gain1, gain2 = 1.0, 1.0
+
+    (result_audio,) = audio_blend_node.blend_audio(audio1_short, gain1, gain2, audio2_long)
+
+    # Assertions
+    assert result_audio['sample_rate'] == sr
+    # Output length should match the longer audio
+    assert result_audio['waveform'].shape[2] == len_long
+
+    # Check the tail end of the blended audio. It should only contain audio2's contribution.
+    tail_start_index = len_short
+    output_tail = result_audio['waveform'][..., tail_start_index:]
+    expected_tail = audio2_long['waveform'][..., tail_start_index:] * gain2
+    assert torch.allclose(output_tail, expected_tail)
+
+
+def test_blend_mismatched_batch_size(audio_blend_node):
+    """
+    Tests blending inputs with different batch sizes. The output batch size should be the max.
+    """
+    sr = 44100
+    b1, b2 = 2, 3
+    samples = 1000
+    channels = 2
+    audio1 = create_dummy_audio(batch_size=b1, channels=channels, samples=samples, sr=sr)
+    audio2 = create_dummy_audio(batch_size=b2, channels=channels, samples=samples, sr=sr)
+    gain1, gain2 = 1.0, 1.0
+
+    (result_audio,) = audio_blend_node.blend_audio(audio1, gain1, gain2, audio2)
+    output_waveform = result_audio['waveform']
+
+    # Assertions
+    # Output batch size should be the max of the two inputs
+    assert output_waveform.shape[0] == max(b1, b2)
+    assert output_waveform.shape[1] == channels
+    assert output_waveform.shape[2] == samples
+
+    # The first two items should be a blend of audio1 and audio2
+    expected_blend_part = (audio1['waveform'] * gain1) + (audio2['waveform'][:b1] * gain2)
+    assert torch.allclose(output_waveform[:b1], expected_blend_part)
+
+    # The last item (index 2) should only contain the contribution from audio2
+    expected_last_item = audio2['waveform'][2] * gain2
+    assert torch.allclose(output_waveform[2], expected_last_item)
diff --git a/source/tests/test_audio_processing_nodes.py b/source/tests/test_audio_processing_nodes.py
new file mode 100644
index 0000000..7fa8285
--- /dev/null
+++ b/source/tests/test_audio_processing_nodes.py
@@ -0,0 +1,230 @@
+"""
+Regression tests for the AudioChannelConverter, AudioResampler, AudioProcessAdvanced node in ComfyUI-AudioBatch.
+"""
+
+import bootstrap  # noqa: F401
+import logging
+import torch
+import pytest
+from nodes.nodes_audio import AudioChannelConverter, AudioResampler, AudioProcessAdvanced
+
+
+# Helper function
+def create_dummy_audio(batch_size, channels, samples, sr, device='cpu'):
+    waveform = torch.randn(batch_size, channels, samples, device=device, dtype=torch.float32)
+    return {"waveform": waveform, "sample_rate": sr}
+
+
+# --- Tests for AudioChannelConverter ---
+
+@pytest.fixture
+def converter_node():
+    return AudioChannelConverter()
+
+
+def test_converter_mono_to_stereo(converter_node):
+    sr = 44100
+    audio_mono = create_dummy_audio(2, 1, 1000, sr)
+    (result_audio,) = converter_node.convert_channels(audio_mono, "mono_to_stereo")
+    assert result_audio['waveform'].shape[1] == 2
+
+
+def test_converter_stereo_to_mono(converter_node):
+    sr = 44100
+    audio_stereo = create_dummy_audio(2, 2, 1000, sr)
+    (result_audio,) = converter_node.convert_channels(audio_stereo, "stereo_to_mono")
+    assert result_audio['waveform'].shape[1] == 1
+
+
+def test_converter_force_stereo_from_multichannel(converter_node):
+    """Tests if force_stereo takes the first channel of a 5.1 input."""
+    sr = 44100
+    audio_5_1 = create_dummy_audio(1, 6, 1000, sr)
+    (result_audio,) = converter_node.convert_channels(audio_5_1, "force_stereo")
+    assert result_audio['waveform'].shape[1] == 2
+    # Check if the two output channels are identical copies of the first input channel
+    assert torch.allclose(result_audio['waveform'][:, 0, :], result_audio['waveform'][:, 1, :])
+    assert torch.allclose(result_audio['waveform'][:, 0, :], audio_5_1['waveform'][:, 0, :])
+
+
+def test_converter_keep_channels(converter_node):
+    sr = 44100
+    audio_stereo = create_dummy_audio(1, 2, 1000, sr)
+    (result_audio,) = converter_node.convert_channels(audio_stereo, "keep")
+    assert result_audio['waveform'].shape[1] == 2
+    assert torch.allclose(result_audio['waveform'], audio_stereo['waveform'])
+
+
+def test_converter_stereo_to_mono_content_is_average(converter_node):
+    """
+    Strictly tests if stereo_to_mono correctly averages the L and R channels.
+    """
+    sr = 44100
+    samples = sr * 1  # 1 second
+
+    # Create distinct L and R channels
+    # A simple ramp for L and a constant for R makes averaging easy to verify
+    left_channel = torch.linspace(0, 1, samples)
+    right_channel = torch.ones(samples) * 0.5
+
+    # Create the stereo waveform: (B=1, C=2, N)
+    stereo_waveform = torch.stack((left_channel, right_channel), dim=0).unsqueeze(0)
+    stereo_audio = {"waveform": stereo_waveform, "sample_rate": sr}
+
+    # Run the conversion
+    (mono_audio,) = converter_node.convert_channels(stereo_audio, "stereo_to_mono")
+
+    # Assertions
+    # The output waveform should be the mathematical average of L and R
+    expected_mono_waveform = ((left_channel + right_channel) / 2.0).unsqueeze(0).unsqueeze(0)  # Shape to (1,1,N)
+
+    assert mono_audio['waveform'].shape == (1, 1, samples)
+    # Use a small tolerance (atol) for floating point comparisons
+    assert torch.allclose(mono_audio['waveform'], expected_mono_waveform, atol=1e-6)
+
+
+# --- Tests for AudioResampler ---
+
+@pytest.fixture
+def resampler_node():
+    return AudioResampler()
+
+
+def test_resampler_upsample(resampler_node):
+    sr_orig, sr_target = 22050, 44100
+    samples_orig = 1000
+    audio = create_dummy_audio(2, 2, samples_orig, sr_orig)
+    (result_audio,) = resampler_node.resample_audio(audio, sr_target)
+
+    expected_samples = int(samples_orig * (sr_target / sr_orig))
+    assert result_audio['sample_rate'] == sr_target
+    assert result_audio['waveform'].shape[2] == expected_samples
+
+
+def test_resampler_downsample(resampler_node):
+    sr_orig, sr_target = 48000, 16000
+    samples_orig = 3000
+    audio = create_dummy_audio(1, 1, samples_orig, sr_orig)
+    (result_audio,) = resampler_node.resample_audio(audio, sr_target)
+
+    expected_samples = int(samples_orig * (sr_target / sr_orig))
+    assert result_audio['sample_rate'] == sr_target
+    assert abs(result_audio['waveform'].shape[2] - expected_samples) < 2  # Resampling can have off-by-one
+
+
+def test_resampler_no_op(resampler_node):
+    """Tests if resampling is skipped when target SR is 0 or same as original."""
+    sr = 44100
+    audio = create_dummy_audio(1, 2, 1000, sr)
+
+    # Test with target_sr = 0
+    (result_audio_zero,) = resampler_node.resample_audio(audio, 0)
+    assert torch.allclose(result_audio_zero['waveform'], audio['waveform'])
+    assert result_audio_zero['sample_rate'] == sr
+
+    # Test with target_sr = original_sr
+    (result_audio_same,) = resampler_node.resample_audio(audio, sr)
+    assert torch.allclose(result_audio_same['waveform'], audio['waveform'])
+    assert result_audio_same['sample_rate'] == sr
+
+
+def get_peak_frequency(waveform: torch.Tensor, sample_rate: int) -> float:
+    """Helper to find the dominant frequency in a waveform using FFT."""
+    if waveform.ndim > 1:
+        # Use the first channel and first batch item for analysis
+        waveform = waveform.squeeze()
+        if waveform.ndim > 1:
+            waveform = waveform[0]
+
+    # Perform Real Fast Fourier Transform
+    fft_result = torch.fft.rfft(waveform)
+    # Get frequency bins for the FFT result
+    freq_bins = torch.fft.rfftfreq(n=waveform.size(-1), d=1./sample_rate)
+    # Find the index of the maximum magnitude in the FFT
+    peak_index = torch.argmax(torch.abs(fft_result))
+    # Get the frequency corresponding to that peak index
+    peak_freq = freq_bins[peak_index].item()
+    return peak_freq
+
+
+def test_resampler_preserves_frequency_content(resampler_node):
+    """
+    Strictly tests if resampling correctly preserves the pitch (frequency) of a signal.
+    """
+    # Original signal: A 440 Hz sine wave at 44100 Hz SR
+    sr_orig = 44100
+    target_freq = 440.0  # A4 note
+    duration = 1.0
+    samples_orig = int(sr_orig * duration)
+    t = torch.linspace(0., duration, samples_orig)
+    original_waveform = torch.sin(2 * torch.pi * target_freq * t).unsqueeze(0).unsqueeze(0)  # (1,1,N)
+    original_audio = {"waveform": original_waveform, "sample_rate": sr_orig}
+
+    # Check the frequency of the original signal to ensure our helper works
+    original_peak_freq = get_peak_frequency(original_waveform, sr_orig)
+    assert abs(original_peak_freq - target_freq) < 1.0  # Allow for slight FFT bin inaccuracy
+
+    # --- Test Downsampling ---
+    sr_down = 16000
+    (downsampled_audio,) = resampler_node.resample_audio(original_audio, sr_down)
+
+    # The frequency content should still be centered at 440 Hz
+    downsampled_peak_freq = get_peak_frequency(downsampled_audio['waveform'], sr_down)
+    logging.info(f"Downsampled from {sr_orig}Hz to {sr_down}Hz. Original peak: {original_peak_freq:.2f}Hz, "
+                 f"New peak: {downsampled_peak_freq:.2f}Hz")
+    assert abs(downsampled_peak_freq - target_freq) < 2.0  # Allow slightly larger tolerance for resamplers
+
+    # --- Test Upsampling ---
+    sr_up = 48000
+    (upsampled_audio,) = resampler_node.resample_audio(original_audio, sr_up)
+
+    # The frequency content should still be centered at 440 Hz
+    upsampled_peak_freq = get_peak_frequency(upsampled_audio['waveform'], sr_up)
+    logging.info(f"Upsampled from {sr_orig}Hz to {sr_up}Hz. Original peak: {original_peak_freq:.2f}Hz, "
+                 f"New peak: {upsampled_peak_freq:.2f}Hz")
+    assert abs(upsampled_peak_freq - target_freq) < 2.0
+
+
+# --- Test for AudioProcessAdvanced ---
+
+@pytest.fixture
+def advanced_processor_node():
+    return AudioProcessAdvanced()
+
+
+def test_advanced_processor_integration_smoke_test(advanced_processor_node):
+    """
+    A simple "smoke test" to ensure AudioProcessAdvanced correctly calls its
+    sub-components and produces an output of the expected shape and SR.
+    We don't need to re-test all permutations, as those are covered
+    by the unit tests for the individual converter and resampler nodes.
+    """
+    # Setup: Start with stereo audio at a high sample rate
+    sr_orig = 44100
+    audio_stereo_orig = create_dummy_audio(1, 2, sr_orig * 2, sr_orig)  # 2 seconds
+
+    # Action: Convert to mono and downsample to 16k
+    target_sr = 16000
+    (result_audio,) = advanced_processor_node.process_audio(
+        audio=audio_stereo_orig,
+        channel_conversion="force_mono",
+        target_sample_rate=target_sr
+    )
+
+    # Assertions:
+    # 1. Did it successfully produce an output?
+    assert result_audio is not None
+    assert "waveform" in result_audio
+    assert "sample_rate" in result_audio
+
+    # 2. Is the final sample rate correct?
+    assert result_audio['sample_rate'] == target_sr
+
+    # 3. Is the final channel count correct?
+    assert result_audio['waveform'].shape[1] == 1  # Should be mono
+
+    # 4. Is the final number of samples roughly correct?
+    original_samples = sr_orig * 2
+    expected_samples = int(original_samples * (target_sr / sr_orig))
+    # Check that the actual number of samples is close to the expected number
+    assert abs(result_audio['waveform'].shape[2] - expected_samples) < 2
diff --git a/source/tests/test_select_audio_from_batch.py b/source/tests/test_select_audio_from_batch.py
new file mode 100644
index 0000000..6dfacfa
--- /dev/null
+++ b/source/tests/test_select_audio_from_batch.py
@@ -0,0 +1,75 @@
+"""
+Regression tests for the SelectAudioFromBatch node in ComfyUI-AudioBatch.
+"""
+
+import bootstrap  # noqa: F401
+import torch
+import pytest
+from nodes.nodes_audio import SelectAudioFromBatch
+
+
+# Helper function
+def create_dummy_audio(batch_size, channels, samples, sr, device='cpu'):
+    waveform = torch.randn(batch_size, channels, samples, device=device, dtype=torch.float32)
+    return {"waveform": waveform, "sample_rate": sr}
+
+
+@pytest.fixture
+def select_node():
+    return SelectAudioFromBatch()
+
+
+def test_select_valid_index(select_node):
+    """Tests selecting a valid index from the batch."""
+    sr = 44100
+    audio_batch = create_dummy_audio(5, 2, 1000, sr)
+
+    (result_audio,) = select_node.select_audio(audio_batch, index=2, behavior_out_of_range="error",
+                                               silence_duration_seconds=1.0)
+
+    # Output should be a batch of 1
+    assert result_audio['waveform'].shape[0] == 1
+    # It should be the 3rd item from the original batch
+    assert torch.allclose(result_audio['waveform'], audio_batch['waveform'][2:3, :, :])
+    assert result_audio['sample_rate'] == sr
+
+
+def test_select_out_of_range_error(select_node):
+    """Tests if an error is raised when index is out of range and behavior is 'error'."""
+    sr = 44100
+    audio_batch = create_dummy_audio(3, 2, 1000, sr)
+
+    with pytest.raises(ValueError, match="Index 5 is out of range for batch of size 3"):
+        select_node.select_audio(audio_batch, index=5, behavior_out_of_range="error", silence_duration_seconds=1.0)
+
+
+def test_select_out_of_range_silence_original_length(select_node):
+    """Tests if silent audio of original length is returned for an out-of-range index."""
+    sr = 44100
+    samples = 1234
+    channels = 2
+    audio_batch = create_dummy_audio(3, channels, samples, sr)
+
+    (result_audio,) = select_node.select_audio(audio_batch, index=3, behavior_out_of_range="silence_original_length",
+                                               silence_duration_seconds=1.0)
+
+    assert result_audio['waveform'].shape == (1, channels, samples)
+    assert result_audio['sample_rate'] == sr
+    # Check if it's all zeros (silence)
+    assert torch.all(result_audio['waveform'] == 0)
+
+
+def test_select_out_of_range_silence_fixed_length(select_node):
+    """Tests if silent audio of a fixed length is returned for an out-of-range index."""
+    sr = 48000
+    audio_batch = create_dummy_audio(3, 1, 1000, sr)
+    silence_duration = 2.5
+
+    (result_audio,) = select_node.select_audio(audio_batch, index=10, behavior_out_of_range="silence_fixed_length",
+                                               silence_duration_seconds=silence_duration)
+
+    expected_samples = int(sr * silence_duration)
+
+    assert result_audio['waveform'].shape == (1, 1, expected_samples)
+    assert result_audio['sample_rate'] == sr
+    assert torch.all(result_audio['waveform'] == 0)