There’s a good post on Apple’s forum about this problem — having no sound when trying to add background music to iDVD — but there was one step that tripped me up. The key concept here is that the problem is that iDVD has the background music set to mute, and you need to un-mute it.
So basically you need to look at the menu drop zone and remove the iTunes note icon. Now look closely at exactly where the note icon just was, and you will see the standard, speaker icon, but it will be black. It’s hard to see so make sure you are looking at exactly where the iTunes note icon was that you just removed. Once you see the speaker icon, use the mouse (or possibly the keyboard) to increase the volume. You should now be able to re-add your background music to the clip and you should be able to hear it.
This was a very frustrating one — so hopefully this tip will save you some time.