I recently acquired a MiniDV camcorder, and suddenly I had an excuse to play with iMovie and iDVD. However, when I burnt my most recent DVD, I discovered the soundtrack I had carefully placed there in iMovie was replaced with a Christmas carol.
I checked the movie I originally dropped onto iMovie, yes it was fine. I checked iDVD's preview mode - still fine. I peeked inside the iDVD document with a text editor, to see if any references to other audio files had crept in - nothing. I toggled the preference that causes iDVD to clear its cache on quit, restarted iDVD, and burnt another DVD - iDVD replaced my sound track with its own favourite carol, once again.
After much messing around and interrupting iDVD in the middle of operation and other such invasive activities, I discovered that iDVD handles the audio track of a movie differently from the video track: while video tracks are encoded in the background and cached in the iDVD document, audio tracks are encoded just prior to burning the DVD, and are stored in a temporary file.
There are three problems with this approach:
Those three problems add up to a ticking timebomb for anyone who wants to use iDVD and also have multiple logins on the one Mac: It turns out that my sister had used the computer to make a Christmas DVD a short while before (hence the choice of a Christmas carol). When she burnt the DVD, iDVD left the carol sitting around as a temporary file. When I went to burn my DVD, iDVD couldn't replace the carol with my soundtrack (since the file was owned by my sister, not me), so it replaced my soundtrack with the carol.
Fortunately, once I'd figured out all this diabolical mechanism, the solution (in Panther, at least) was easy:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040111064120832