Event.
Page 1 right side 4 from the bottom should be Change system bgm
type of bgm = battle bgm = [your pick]
NEXT battle will be set to these settings.
In battle change music.
right side 8 down should be play BGM
Choose your midi and hit ok.
if you made a trigger on ether event or battle page, you might want to check that the command of the change of music setting is being hit, if not then it could be that the maker is missing that command and playing what it knows.
if problems still occur let us know.