MP3 encoding in Audio Magic is performed by LAME, via lamejs.

  lamejs   https://github.com/zhuker/lamejs
           (fork used: @breezystack/lamejs 1.2.7,
            https://github.com/shijinyu/lamejs)
  LAME     https://lame.sourceforge.net

lamejs is a JavaScript rewrite of jump3r-code, itself a rewrite of
libmp3lame. It is licensed under the GNU Lesser General Public License,
version 3.0 (LGPL-3.0): https://www.gnu.org/licenses/lgpl-3.0.html

Audio Magic uses lamejs unmodified. It is loaded as a separate script at
runtime, only when a file is converted to MP3, and is not combined into
the application bundle — so it can be substituted with another build of
the library.

The upstream LICENSE file, reproduced verbatim:

--------------------------------------------------------------------
Can I use LAME in my commercial program?  

Yes, you can, under the restrictions of the LGPL.  The easiest
way to do this is to:

1. Link to LAME as separate jar (lame.min.js or lame.all.js)

2. Fully acknowledge that you are using LAME, and give a link
   to our web site, lame.sourceforge.net

3. If you make modifications to LAME, you *must* release these
   these modifications back to the LAME project, under the LGPL.
--------------------------------------------------------------------