iOS Safari can't decode your .mov, and the reason is 2 bytes deep in the container

Our tool transcribes audio in the browser — Whisper running locally via transformers.js , no upload. It worked fine, until analytics showed something too clean to be a coincidence: .mov uploads on mobile failed 100% of the time. Not 90%. Every single one. Desktop had never reported a single .mov failure. This is what I found, and how it got fixed without pulling in ffmpeg.wasm or WebCodecs. The 30-second reproduction I took one AAC audio track and put it in two containers — same encoder, same bytes for the audio itself, only the wrapper differs: const buf = await file . arrayBuffer (); await new AudioContext (). decodeAudioData ( buf ); On an iPhone 17 Pro simulator (iOS 18.7 / Safari 26.5): File iOS Safari Chromium sample.mov (ftyp qt ) EncodingError: Decoding failed OK sample.mp4 (ftyp isom ) OK OK So it isn't the codec. It's the container. The obvious fix that doesn't work First instinct: it's the brand in the ftyp box. Patch qt → isom , four bytes, done. It still fails. I'm writing this down so nobody else burns an afternoon on it. The ftyp brand is not what Safari looks at. The difference lives inside moov . The actual root cause Dig down to moov → trak → mdia → minf → stbl → stsd — the sample description that tells the decoder how the audio is encoded. Both files carry an mp4a entry. They are not the same mp4a entry: QuickTime writes: MP4 expects: version = 1 <— version = 0 compressionID = -2 (fffe) <— compressionID = 0 + 16 bytes of v1 extension <— (absent) esds wrapped in a 'wave' box <— esds is a direct child extra 'chan' channel layout (absent) iOS Safari's decodeAudioData only accepts a version 0 audio sample entry. Chromium accepts both — which is exactly why desktop never saw this and mobile never survived it. That version field is a uint16 . Two bytes decide whether the file plays. The fix: rebuild the container, don't touch the codec Since the audio bitstream is already valid AAC, nothing needs to be re-encoded. The job is pure byte plumbing: extract