diff --git a/tests/test_domain_audio_buffer_extras.py b/tests/test_domain_audio_buffer_extras.py
index 430b838..29b8de8 100644
--- a/tests/test_domain_audio_buffer_extras.py
+++ b/tests/test_domain_audio_buffer_extras.py
@@ -2,58 +2,75 @@
 
 import numpy as np
 import pytest
+from unittest.mock import patch
 
 from abogen.domain.audio_buffer import fit_audio_to_duration, ffmpeg_time_stretch, SAMPLE_RATE
 
 
 class TestFitAudioToDuration:
-    def test_exact_length(self):
-        audio = np.ones(24000, dtype="float32")
-        result = fit_audio_to_duration(audio, 1.0, SAMPLE_RATE)
-        assert len(result) == 24000
+  def test_exact_length(self):
+    audio = np.ones(24000, dtype="float32")
+    result = fit_audio_to_duration(audio, 1.0, SAMPLE_RATE)
+    assert len(result) == 24000
 
-    def test_shorter_pads_with_zeros(self):
-        audio = np.ones(12000, dtype="float32")
-        result = fit_audio_to_duration(audio, 1.0, SAMPLE_RATE)
-        assert len(result) == 24000
-        assert result[0] == 1.0
-        assert result[12000] == 0.0
+  def test_shorter_pads_with_zeros(self):
+    audio = np.ones(12000, dtype="float32")
+    result = fit_audio_to_duration(audio, 1.0, SAMPLE_RATE)
+    assert len(result) == 24000
+    assert result[0] == 1.0
+    assert result[12000] == 0.0
 
-    def test_longer_trims(self):
-        audio = np.ones(48000, dtype="float32")
-        result = fit_audio_to_duration(audio, 1.0, SAMPLE_RATE)
-        assert len(result) == 24000
-        assert result[-1] == 1.0
+  def test_longer_trims(self):
+    audio = np.ones(48000, dtype="float32")
+    result = fit_audio_to_duration(audio, 1.0, SAMPLE_RATE)
+    assert len(result) == 24000
+    assert result[-1] == 1.0
 
-    def test_empty_input(self):
-        result = fit_audio_to_duration(np.array([], dtype="float32"), 0.5, SAMPLE_RATE)
-        assert len(result) == 12000
-        assert np.all(result == 0.0)
+  def test_empty_input(self):
+    result = fit_audio_to_duration(np.array([], dtype="float32"), 0.5, SAMPLE_RATE)
+    assert len(result) == 12000
+    assert np.all(result == 0.0)
 
-    def test_output_dtype(self):
-        audio = np.ones(100, dtype="float32")
-        result = fit_audio_to_duration(audio, 0.5, SAMPLE_RATE)
-        assert result.dtype == np.float32
+  def test_output_dtype(self):
+    audio = np.ones(100, dtype="float32")
+    result = fit_audio_to_duration(audio, 0.5, SAMPLE_RATE)
+    assert result.dtype == np.float32
 
 
 class TestFfmpegTimeStretch:
-    def test_no_stretch_below_threshold(self):
-        audio = np.ones(24000, dtype="float32")
-        result = ffmpeg_time_stretch(audio, 0.8, SAMPLE_RATE)
-        np.testing.assert_array_equal(result, audio)
+  def test_no_stretch_below_threshold(self):
+    audio = np.ones(24000, dtype="float32")
+    result = ffmpeg_time_stretch(audio, 0.8, SAMPLE_RATE)
+    np.testing.assert_array_equal(result, audio)
 
-    def test_no_stretch_at_exactly_one(self):
-        audio = np.ones(24000, dtype="float32")
-        result = ffmpeg_time_stretch(audio, 1.0, SAMPLE_RATE)
-        np.testing.assert_array_equal(result, audio)
+  def test_no_stretch_at_exactly_one(self):
+    audio = np.ones(24000, dtype="float32")
+    result = ffmpeg_time_stretch(audio, 1.0, SAMPLE_RATE)
+    np.testing.assert_array_equal(result, audio)
 
-    def test_empty_audio(self):
-        result = ffmpeg_time_stretch(np.array([], dtype="float32"), 2.0, SAMPLE_RATE)
-        assert len(result) == 0
+  def test_empty_audio(self):
+    result = ffmpeg_time_stretch(np.array([], dtype="float32"), 2.0, SAMPLE_RATE)
+    assert len(result) == 0
 
-    def test_stretch_reduces_duration(self):
-        audio = np.random.randn(48000).astype("float32")
-        result = ffmpeg_time_stretch(audio, 2.0, SAMPLE_RATE)
-        assert len(result) < len(audio)
-        assert len(result) > 0
-        assert result.dtype == np.float32
+@patch("subprocess.Popen")
+def test_stretch_reduces_duration(mock_popen):
+  # Mock subprocess response
+  mock_proc = mock_popen.return_value
+  mock_proc.returncode = 0
+  mock_proc.communicate.return_value = (b"\x00" * 400, b"")
+  mock_proc.stdout.read.return_value = b"\x00" * 400
+
+  audio = np.random.randn(48000).astype("float32")
+  result = ffmpeg_time_stretch(audio, 2.0, SAMPLE_RATE)
+
+  # Verify ffmpeg was called with correct args
+  mock_popen.assert_called_once()
+  args = mock_popen.call_args[0][0]
+  assert "ffmpeg" in args[0]
+  assert "-filter:a" in args
+  assert any("atempo=" in arg for arg in args)
+
+  # Verify result has reduced duration and correct dtype
+  assert len(result) < len(audio)
+  assert len(result) > 0
+  assert result.dtype == np.float32