Justility uses unmodified @breezystack/lamejs 1.2.7, a JavaScript port of LAME by Alex Zhukov and contributors, under LGPL-3.0. LAME: https://lame.sourceforge.net/
The encoder is the separate module lamejs.js in this directory. Source and build configuration: lamejs-source.zip (published revision 1fb0ef5fa177413107e2e107d054a9b994e3f79c). Full license texts: LGPL-3.0.txt and GPL-3.0.txt; original notice: lamejs-LICENSE.txt.
Replacement: in a local/self-hosted Justility checkout, replace public/vendor/creative-audio/lamejs.js with a compatible ES module exporting Mp3Encoder (constructor channels, sampleRate, bitrate; encodeBuffer and flush). Serve that checkout without running prepare:vendor again, which restores the shipped module. The adapter lives separately in src/tools/creative/audio-worker.ts and requires no encoder modifications. No technical restriction prevents modification, replacement or debugging of the library.