For example there are 5 game tracks and 2 riichi tracks. To add them move them to assets/audio/bg and name them riichix.mp3 or game.mp3. Then add them to extension.json (just follow the ones there, and insert them 1 each line) and to script.js (change the riichiN and gameN values to the number of tracks you have)
vg-mjg/Random-BGM-majsoul
Makes majsoul play random songs from a list of tracks. It affects riichi and bgm.
JavaScript