Use a system NPC to play the music for each level.
PHP Code:
this.midi = {"stef3", "stef23", "stef7"};
this.midi.stef3 = {"level2.nw", "level3.nw", "level4.nw"};
this.midi.stef23 = {"level5.nw", "level6.nw", "level7.nw", "level8.nw"};
this.midi.stef7 = {"level9.nw", "level10.nw"};
for (a: this.midi) {
if (makevar("this.midi." @ a).index(player.level) >= 0) playlooped(a @ ".mid");
}