To put in custom music, you do this first:
1. Make yer music, you know, yadda yadda yadda...
2. Open up your level
3. Once opened, go to Build Game (Trust me, I've done this plenty.)
4. Go to Level Settings at the top of the Build Game window.
5. Click on EDIT on the Global Script column.
6. Under GENERAL, click on the "..." thing by the music file it shows.
7.Choose your song, sound, or whatever, click OK.
8. Then click OK on the Build Game Menu.
Doing this will not make your game, but will return it to how it was, with the changes you made.
Cool Stuff Software is gnarley.