I know this is simple to do but ive never seen a guide for it so here
Steps
1. Go to GAME FOLDER > Music
2. Double click the files to see which song is for which map
3. The Song that you want to be for that map rename it to that maps song file #
EXAMPLE > 1000-4 is Cronus Song , so i would take my mp3 file and rename it to 1000-4 and put it in GAME FOLDER > Music
Give a Thanks if i helped any =D