[Bugfix] Fix scipy audio resampling ratio (#42233)

Signed-off-by: JooHo Lee <BWAAEEEK@users.noreply.github.com>
Co-authored-by: JooHo Lee <BWAAEEEK@users.noreply.github.com>

tests/multimodal/test_audio.py

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 # test_audio.py
+import math
 from unittest.mock import patch
 
 import numpy as np
@@ -45,7 +46,6 @@ def test_resample_audio_scipy(dummy_audio):
     assert np.all(out_same == dummy_audio)
 
 
-@pytest.mark.xfail(reason="resample_audio_scipy is buggy for non-integer ratios")
 def test_resample_audio_scipy_non_integer_ratio(dummy_audio):
     out = resample_audio_scipy(dummy_audio, orig_sr=5, target_sr=3)
 
@@ -56,6 +56,26 @@ def test_resample_audio_scipy_non_integer_ratio(dummy_audio):
     assert np.isfinite(out).all()
 
 
+def test_resample_audio_scipy_non_divisible_sample_rates():
+    audio = np.arange(441, dtype=float)
+    out = resample_audio_scipy(audio, orig_sr=44100, target_sr=16000)
+
+    expected_len = math.ceil(len(audio) * 16000 / 44100)
+    assert len(out) == expected_len
+
+    assert isinstance(out, np.ndarray)
+    assert np.isfinite(out).all()
+
+
+def test_resample_audio_scipy_resamples_last_axis_for_multichannel():
+    audio = np.arange(2 * 441, dtype=float).reshape(2, 441)
+    out = resample_audio_scipy(audio, orig_sr=44100, target_sr=16000)
+
+    expected_len = math.ceil(audio.shape[-1] * 16000 / 44100)
+    assert out.shape == (2, expected_len)
+    assert np.isfinite(out).all()
+
+
 def test_audio_resampler_pyav_calls_resample(dummy_audio):
     resampler = AudioResampler(target_sr=22050, method="pyav")
     with patch("vllm.multimodal.audio.resample_audio_pyav") as mock_resample:

vllm/multimodal/audio.py

@@ -230,11 +230,19 @@ def resample_audio_scipy(
     orig_sr: float,
     target_sr: float,
 ) -> npt.NDArray[np.floating]:
-    if orig_sr > target_sr:
-        return scipy_signal.resample_poly(audio, 1, orig_sr // target_sr)
-    elif orig_sr < target_sr:
-        return scipy_signal.resample_poly(audio, target_sr // orig_sr, 1)
-    return audio
+    orig_sr_int = int(round(orig_sr))
+    target_sr_int = int(round(target_sr))
+
+    if orig_sr_int == target_sr_int:
+        return audio
+
+    gcd = math.gcd(orig_sr_int, target_sr_int)
+    return scipy_signal.resample_poly(
+        audio,
+        target_sr_int // gcd,
+        orig_sr_int // gcd,
+        axis=-1,
+    )
 
 
 class AudioResampler: