This should really have been placed in the Use Map Settings Forum, but, here we go.
You can set up a map to play your music.
Convert your music to .wav , I think the editor likes 16 bit, why I don't know, 8 bit condences, and still sounds resonably ok.
Set up a trigger,,,
All Players
Condition
Always
Action
Play Wave file (Choose your music for this)
Wait ???????????? milliseconds (however long the music plays)
Preserve trigger
1000 milliseconds = 1 second
Be sure to tell all that are playing to turn off the background music so that all they hear is the music you chose.
Now,, are you ready for the HUGE drawback....
Your little 1 meg mp3 is now about 20 meg, and with the compression in the ScEditor, it will be maybe 15 meg in the game.
Unless it's a one person map, or you and your friends are cable or T1 plus, or have more patience then the average teenager, it will be a very l-o-n-g download.