Beatseam's MP3 encoder uses LAME (via lamejs), licensed under the GNU Lesser General Public License v3. The unmodified library file is served separately at /vendor/lame.min.js. LAME project: lame.sourceforge.io. lamejs: github.com/zhuker/lamejs.
Fonts: Michroma, Figtree and DM Mono from Google Fonts, under the SIL Open Font License.