I've heard the question posed before: "How do I replace the current in-game music with my own?" Well, here's how:

Go to the default install directory of HZ, and look under Resources/sound/music (on my machine it is C:\Program Files\Artifact Entertainment\Horizons\Resources\sound\music). It should not come as a surprise that all of the in-game music is stored here. Feel free to browse through and listen to the music. To help make things easier for you all, the names of the most-commonly used music in the game are as follows...

Battle Music:
battle_X_loop.mp3 where X ranges from 1 to 7.

Generic ambient music:
dungeon_4_loop.mp3
dungeon_mus_01.mp3
dungeon_mus_02.mp3
dungeon_mus_03.mp3
dungeon_mus_05.mp3
mus_gen_in_darkness.mp3
nowhere_mus.mp3

Loading theme music:
theme_loading.mp3

Make note of the filenames for the music you wish to replace. Select the .MP3s you want to use as replacements, and copy them over to your resources_override folder. Rename these MP3s to exactly match those of the in-game music you want to replace.

That's it! Next time you play the game, the client will use the .MP3s from the resources_override folder, that is, the music you want to listen to.