MP3 export in the Audio Morpher uses lamejs 1.2.1 (lame.min.js), a JavaScript port of the LAME MP3 encoder by Alex Zhukov (https://github.com/zhuker/lamejs), licensed under the GNU Lesser General Public License version 3 (LGPL-3.0).
The file lame.min.js is unmodified and is loaded as a separate script, so it can be replaced by any other build of lamejs. The full licence texts are in lamejs-LICENSE-LGPL-3.0.txt and lamejs-LICENSE-GPL-3.0.txt (the LGPL refers to the GPL). Source code: https://github.com/zhuker/lamejs (npm package lamejs@1.2.1).