This is actually something you can do yourself. This isn't a comphrensive guide, you'll have to use trial and error for it, to know what sounds to replace.
Step 1 : Figure out what you want to listen to while you play. I use some Two Steps from Hell for mine, some want some dub-step, or AC/DC, or w/e.
Step 2 : Convert the music you want over to .ogg files. You can use this :
http://download.cnet.com/Free-MP3-WMA-OGG-Converter/3000-2140_4-10793572.html .
Step 3
a) Backup your Warband music files.
b) find the songs in the cRPG music.txt file that you will be replacing. These are some I used with good results.
outdoor_beautiful_land.ogg | encounter_hostile_nords.ogg | crazy_battle_music.ogg | town_vaegir.ogg (and the other factions) | fight_as_nord.ogg (and the other factions) | lords_hall_nord.ogg (and the other factions) | tavern_1.ogg (and tavern_2) | calm_night.ogg
c) rename your NEW .ogg files with the names of the ones you are replacing (from step 3.b.)
d) copy paste them into the MAIN Warband Music folder. Choose to replace files when prompted.
The way this works, is the cRPG text file will "tell" crpg to fetch certain songs from the Warband folder. When it fetches those songs, you'll hear your custom music playing instead!
EDIT : Also, hey everyone! Long time no see!