There is a way to add custom music, but it's not very elegant.
I did it like this:
Let's say you want to add custom music for battles. In music.txt you can see:
fight_1.ogg 5120 5120
fight_2.ogg 5120 5120
fight_3.ogg 5120 5120
fight_as_khergit.ogg 5124 5183
fight_as_nord.ogg 5128 5183
fight_as_rhodok.ogg 5136 5183
fight_as_vaegir.ogg 5122 5183
fight_while_mounted_1.ogg 5120 5120
fight_while_mounted_2.ogg 5120 5120
The names of the files are self-explanatory so you can take the numbers that define when that song is played and use it with your own files.
I wanted to add this file:
PrideorPain.mp3
And it looked like this:
PrideorPain.mp3 5120 5120
PrideorPain.mp3 5128 5183
I took the 5120 5120 index which is used for all fights and the 5128 5183 index which is used for nords (that I'm currently playing).
Also, don't forget to change the number at the beginnning of the file to reflect the new number of files in the text.