MP4 to MP3

192 kbps, the default, is a good fit for music taken from a video. The audio in a video is usually already compressed at about 128–256 kbps, so going above the source rarely sounds better; 320 kbps only makes the file larger. For speech, such as lectures or podcasts, 128 kbps mono is clear, and the file is a third smaller than at 192 kbps.

The audio is decoded by your browser's own Web Audio engine, and what it can read varies. AAC audio in an MP4 file, which is what phones and most video sites produce, decodes in current Chrome, Edge, Firefox and Safari. Files fail when they have no audio track, are copy-protected, use a codec the browser lacks, such as Dolby AC-3 in Chrome, or use a container the browser cannot open, which happens with some MOV and MKV files. The failed file is marked with the reason, and trying it in a different browser often helps.

Slightly. The audio in a video file is almost always already compressed, so it is decoded and compressed again as MP3. At 192 kbps or above the difference is not audible for most listeners. Output is always 44.1 kHz, the CD sample rate, and 48 kHz video audio is resampled by the browser.

It is folded down to stereo, or to mono if you pick mono, using the standard Web Audio speaker mix. The centre channel, where film dialogue usually sits, is kept in both sides rather than dropped. If a file is already mono, it is saved as mono even when stereo is selected, because a two-channel copy of one channel only wastes space.

There is no fixed limit, but memory is the constraint. The whole file is read into memory, and the decoded audio takes about 21 MB per minute of stereo sound. A 30-minute video is fine on a laptop. Files over about 500 MB get a warning, and hour-long recordings may fail on phones. Converting one long file at a time helps.