I figured since there's no tutorial for this and it's quite easy, might as well I write one.

HOW TO ADD YOUR OWN MUSIC IN RTW

First of all, you don't need to extract the music pack at all. Just leave it there.
What you need is a folder Data/sounds/music.
Now dump your mp3s in the music folder. NOTE: the files MUST be in mp3 format. I have tried .wma and it doesn't work.
Now go to your Data folder and open this file: desc_sounds_music.txt
Just replace or simply add the names of your music files in the appropriate event lines. It's pretty self explanatory.


LAST NOTE: If you've never modded the music before and you've just upgraded with 1.2 patch, there's a chance you have a file in Data/sounds folder called events.dat. This file is not really used by the game but it does overwrite any changes you made to desc_sounds_music.txt so if you find this file, just rename or dump it.