Here is something that may help.

1) Find a song you like.

2) You need to convert the song from it's original format (cd audio, mp3, WMA, etc.) into a WAV file format. Personally I use a program called CDex from http://cdexos.sourceforge.net/

3) You'll need a WAV file editor if you want to shorten the song, adjust the volume, etc. I use the WAve Editor that comes with Ahead's Nero 5.5

4) Save your WAV file into 22050hz 16bit mono as Goresh said. Again, I use the Nero Wave Editor.

5) Convert the WAV file into a SFX sound file the game can recognize using the Heat tool called WinResToolsWizard, found on this site.

6) Once you get your song into SFX format (make sure it's named SONG.SFX), you'll need to put it into the UI.RES file.
Again using the WinResToolsWizard.

7) Using WinResToolsWizard, unpack the UI.RES file into a temporary folder, replace the old SONG.SFX file with yours, and then repack the UI.RES using WinResToolsWizard.

DONE... PLAY GAME, NEW SONG! Yea! .

This was put together by someone else. Can't remember who. Same principle for engine sounds.
