Here is a sample mod that let's you put in custom music (must be in .ogg format).
Just extract the archive into the game's mods folder (C:\Program files (x86)\Steam\steamapps\common\Horizon\mods) and replace the .ogg files with your own. Then enable it in game from the mods manager.
The sample mod replaces just a couple of the music files, if you want you can add the combat ones to be replaced also just put new files with the same names as the ones you want replaced from the main music folder.
Custom Music Mod LINK
An easy to use ogg converter LINK
Sample Mod - How to replace music files with your own custom
Sample Mod - How to replace music files with your own custom
Horizon - Lead Designer | a.k.a. Raf