fix: fall back to soundfile when torchcodec FFmpeg libs are missing

Recent torchaudio defaults to torchcodec as the audio backend, which requires
FFmpeg shared libraries. Falls back to soundfile for envs where torchcodec
can't load (e.g. containerised ComfyUI without system FFmpeg).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nodes/selva_lora_trainer.py

@@ -51,7 +51,15 @@ def _find_audio(npz_path: Path) -> Path | None:
 
 
 def _load_audio(path: Path, target_sr: int, duration: float) -> torch.Tensor:
-    waveform, sr = torchaudio.load(str(path))
+    try:
+        waveform, sr = torchaudio.load(str(path))
+    except RuntimeError as e:
+        if "torchcodec" not in str(e).lower() and "libtorchcodec" not in str(e).lower():
+            raise
+        # torchcodec unavailable (FFmpeg shared libs missing) — fall back to soundfile
+        import soundfile as sf
+        data, sr = sf.read(str(path), always_2d=True)  # [frames, channels]
+        waveform = torch.from_numpy(data.T).float()    # [channels, frames]
     if waveform.shape[0] > 1:
         waveform = waveform.mean(0, keepdim=True)
     waveform = waveform.squeeze(0).float()