Whilst creating a DVD with iDVD, I encountered an issue. When I added music to the audio tracks, the submenus would play the files just fine, while the main (title) window would not.
A quick forum search at Apple Support Discussions led me to find that the problem was most likely due to the audio track being muted without one's knowing it. The solution: remove all the files so that one can see the audio file drop box in the menu inspector; if there are no waves coming out of it, that means it is muted and should be able to be fixed by simply clicking the speaker icon.
However, in some themes seem to have problems with this. The solution beyond this gets slightly more complicated. When you save the iDVD project, it creates a .dvdproj file, which is a file containing resources. Right click this file, and click "Show Package Contents," which brings up the contents of this file. Under this you follow Contents -> Resources -> Project Data and open that file. (If you have the Apple Developer's Tools, you have the XML reader so that you can view and edit this much easier.)
In the Property List Editor, find the section labelled "Doom of Fate Jump Targets." Under this, there are several random-looking strings of numbers and letters, and under one of these will be the menu with muted audio. There is a selection labelled "Menu Audio Muted" which will read "Yes," and should be changed to "No."
Save the file, reopen your iDVD project, and it should be fixed! I tried it while the project was still open, and thus it did not work the first time for me. If it doesn't work the first time, try saving and quitting iDVD, editing the file, and then reopening it.