Save output WAVs via soundfile instead of torchaudio.save

torchaudio 2.11 removed its old save backends in favor of torchcodec,
which isn't installed, so torchaudio.save() raised ImportError on the
very last step of every generation. soundfile is already a pinned
dependency and needs no extra backend.

app.py

@@ -26,6 +26,7 @@ from pathlib import Path
 import torch
 import numpy as np
 import torchaudio
+import soundfile as sf
 import ffmpeg
 import spaces
 import gradio as gr
@@ -1039,11 +1040,10 @@ def _stitch_wavs(wavs: list[np.ndarray], crossfade_s: float, db_boost: float,
 
 
 def _save_wav(path: str, wav: np.ndarray, sr: int) -> None:
-    """Save a numpy wav array to *path* as stereo (2, T) via torchaudio.
+    """Save a numpy wav array to *path* as stereo (2, T) via soundfile.
     Always conforms to stereo so every output WAV is a proper 2-channel file."""
     wav = _to_stereo(wav)   # (1,T) or (T,) → (2,T); (2,T) is a no-op
-    t = torch.from_numpy(np.ascontiguousarray(wav))
-    torchaudio.save(path, t, sr)
+    sf.write(path, np.ascontiguousarray(wav.T), sr)
 
 
 def _log_inference_timing(label: str, elapsed: float, n_segs: int,