I'd likely do something like this:
// Call this before the battle with the boss starts.
void main()
{
object oArea = GetArea (OBJECT_SELF);
int nBattle = MusicBackgroundGetBattleTrack (oArea);
// Store it on the area for reverting later.
SetLocalInt (oArea, "NORMAL_BATTLE_TRACK", nBattle);
int nTrack = 1; // Track number to use here.
MusicBattleChange (oArea, nTrack);
}
// Call this routine from the bosses OnDeath script, or just copy those three lines into it.
void RevertMusic ()
{
object oArea = GetArea (OBJECT_SELF);
int nTrack = GetLocalInt (oArea, "NORMAL_BATTLE_TRACK");
MusicBattleChange (oArea, nTrack);
}