This code works on arcade mode, the music will be played if your facing the char where the song is assigned to. But you still have to assign a song on the stage it self otherwise you'll not gonna be having a bgm during your fight in versus mode.
EX:
[Ryu, stages/suzaku_castle.def, order=1, music=sound/mysong.mp3] = mysong will play if you face Ryu on the arcade
suzaku_castle.def
[bgmusic = sound/exsong.mp3] = exsong will play if you choose that stage in versus, training, or demo mode