This uses the libmp3lame library API. IE: just the encoding part of the lame
package.
Please refer to the detail here: license detail, reproduced below:

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 library (libmp3lame.a on unix or
lame_enc.dll on windows)

2. Fully acknowledge that you are using LAME, and give a link
to our web site, www.mp3dev.org

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