This page performs all media processing locally, without uploading files.

MP3: lamejs 1.2.1 / LAME, unchanged independent library (LGPL).
https://github.com/zhuker/lamejs
https://lame.sourceforge.net/
Source: https://registry.npmjs.org/lamejs/-/lamejs-1.2.1.tgz
See LICENSE-lamejs.txt. Library is fetched only when MP3 is requested.

M4A: native browser AAC encoder, packaged using unchanged mp4-muxer 5.2.2 (MIT).
https://github.com/Vanilagy/mp4-muxer
Source: https://registry.npmjs.org/mp4-muxer/-/mp4-muxer-5.2.2.tgz
See LICENSE-mp4-muxer.txt. Library is fetched only when M4A is requested.
The small pinned muxer is not used to decode untrusted input; browsers decode inputs.