You can save youself the trouble, Red. If you make new music/music.cfg file and put it in a pk3 (which comes later in the alphabet than wq3_pak0), it will probably override the other one. You could add your song to the list of songs to be selected randomly between. To get just the track you want, it probably has to be the only one mentioned in music.cfg, and then it would be played on all the maps.
But anyway, the format of the file is as follows:
MUSICFILE
<number of tracks, max 10 right now>
music/filename.wav <length in msecs> <repeat count>
Just to save you some guesswork.