I modded the sound_music.def file so that you would have 20 (+1 boss music slot) slots to put whatever music you want. Rock sounds best IMO, I have various Disturbed tracks on mine. The music files need to be mp3's or I'm not sure if they will work, and if they do would require you to edit the def file. If anyone knows how to additionally make the game music so that there is a delay between ending the battle and the music ending (so that you can go from monster to monster without the tracks ending) I would be grateful.
You have to rename the music files:
battle_1_loop.mp3
battle_2_loop.mp3
battle_3_loop.mp3
battle_4_loop.mp3
battle_5_loop.mp3
battle_6_loop.mp3
battle_7_loop.mp3
battle_8_loop.mp3
battle_9_loop.mp3
battle_10_loop.mp3
battle_11_loop.mp3
battle_12_loop.mp3
battle_13_loop.mp3
battle_14_loop.mp3
battle_15_loop.mp3
battle_16_loop.mp3
battle_17_loop.mp3
battle_18_loop.mp3
battle_19_loop.mp3
battle_20_loop.mp3
Unzip the attachment into your Horizons folder in Program Files. It'll automatically create all the necessary paths. You just have to add music. To revert to the original music, just delete the folder.