fix: support M4A and other audio formats via ffprobe fallback

app.py

@@ -297,9 +297,23 @@ def _safe_prompt_wav_recognition(
 
 def _get_audio_duration_seconds(audio_path: str) -> float:
     import soundfile as sf
-
-    info = sf.info(audio_path)
-    return float(info.frames) / float(info.samplerate)
+    try:
+        info = sf.info(audio_path)
+        return float(info.frames) / float(info.samplerate)
+    except Exception:
+        pass
+    import subprocess
+    try:
+        result = subprocess.run(
+            ["ffprobe", "-v", "error", "-show_entries", "format=duration",
+             "-of", "default=noprint_wrappers=1:nokey=1", audio_path],
+            capture_output=True, text=True, timeout=10,
+        )
+        if result.returncode == 0 and result.stdout.strip():
+            return float(result.stdout.strip())
+    except (FileNotFoundError, subprocess.TimeoutExpired, ValueError):
+        pass
+    raise RuntimeError(f"Cannot determine audio duration: {audio_path}")
 
 
 def _validate_reference_audio_duration(