read audio fps too

closes #1754

src/App.jsx

@@ -311,7 +311,7 @@ const App = memo(() => {
   }, [seekRel, zoomedDuration]);
 
   const shortStep = useCallback((direction) => {
-    // If we don't know fps, just assume 30 (for example if audio file)
+    // If we don't know fps, just assume 30 (for example if unknown audio file)
     const fps = detectedFps || 30;
 
     // try to align with frame
@@ -1492,8 +1492,15 @@ const App = memo(() => {
 
       // throw new Error('test');
 
+      // eslint-disable-next-line no-inner-declarations
+      function getFps() {
+        if (haveVideoStream) return getStreamFps(videoStream);
+        if (haveAudioStream) return getStreamFps(audioStream);
+        return undefined;
+      }
+
       if (timecode) setStartTimeOffset(timecode);
-      setDetectedFps(haveVideoStream ? getStreamFps(videoStream) : undefined);
+      setDetectedFps(getFps());
       if (!haveVideoStream) setWaveformMode('big-waveform');
       setMainFileMeta({ streams: fileMeta.streams, formatData: fileMeta.format, chapters: fileMeta.chapters });
       setMainVideoStream(videoStream);

src/ffmpeg.js

@@ -523,12 +523,36 @@ export function isProblematicAvc1(outFormat, streams) {
   return isMov(outFormat) && streams.some((s) => s.codec_name === 'h264' && s.codec_tag === '0x31637661' && s.codec_tag_string === 'avc1' && s.pix_fmt === 'yuv422p10le');
 }
 
-export function getStreamFps(stream) {
+function parseFfprobeFps(stream) {
   const match = typeof stream.avg_frame_rate === 'string' && stream.avg_frame_rate.match(/^([0-9]+)\/([0-9]+)$/);
-  if (stream.codec_type === 'video' && match) {
+  if (!match) return undefined;
   const num = parseInt(match[1], 10);
   const den = parseInt(match[2], 10);
   if (den > 0) return num / den;
+  return undefined;
+}
+
+export function getStreamFps(stream) {
+  if (stream.codec_type === 'video') {
+    const fps = parseFfprobeFps(stream);
+    return fps;
+  }
+  if (stream.codec_type === 'audio') {
+    if (typeof stream.sample_rate === 'string') {
+      const sampleRate = parseInt(stream.sample_rate, 10);
+      if (!Number.isNaN(sampleRate) && sampleRate > 0) {
+        if (stream.codec_name === 'mp3') {
+          // https://github.com/mifi/lossless-cut/issues/1754#issuecomment-1774107468
+          const frameSize = 1152;
+          return sampleRate / frameSize;
+        }
+        if (stream.codec_name === 'aac') {
+          // https://stackoverflow.com/questions/59173435/aac-packet-size
+          const frameSize = 1024;
+          return sampleRate / frameSize;
+        }
+      }
+    }
   }
   return undefined;
 }