Extract the audio from an MP4 video without uploading it

A filmed talk you want to listen to while walking, a sound take to recover from a rush. The sound is already there, compressed, inside the file: we copy it out, we do not rebuild it.

What this tool does

NO RE-ENCODING: the bytes of the sound come out exactly as they were inside the video. No second compression, so no added loss.
It is instant, even on a one-gigabyte file: there is nothing to decode, only tables to read and bytes to copy.
The file produced is a .m4a, because that is what it contains. Calling it .mp3 would lie about its format, and half the players would refuse it.
The video's metadata does not follow: not the date, not the device, not the location.

What it does not do: This is NOT a conversion to MP3: converting would mean decoding then re-encoding, therefore degrading — and an MP3 encoder is a dependency this site does not carry. A .m4a opens in VLC, iTunes, Windows Media Player and on every phone. MP4, M4V and MOV files are read; MKV, AVI and WebM are not. Past two gigabytes the file is refused on drop rather than killing the tab.

Nothing is uploaded. Cut your network connection once this page has loaded: the tool keeps working. How to check it yourself.

The other tools

Merge PDFs: Several files, one document, in the order you decide.
Organize pages: Reorder, rotate, delete, duplicate: the whole plan.
Extract pages: Pick pages, take them out into a document of their own.
Delete pages: Remove pages while looking at them, and they do not survive under the file you get.