I've just spent a couple of days (and a stack of blank DVDS) solving this mystery, so I thought I'd share the solution.
I burned a DVD with several tracks. On one of them the audio did not work. It was not totally absent; when I turned the volume very loud I could "sense" it. But on preview in DVDA it worked fine.
Well, I had a phase problem. This was a mono soundtrack being treated as stereo. When the DVD track played on my (mono) TV set, the two channels canceled each other out. So I went back to Vegas, rightclicked the audio, selected Channels and Left only, re-rendered again. Burned again. All well.
So it's a good test: Open Channels, select Combine. If the sound disappears your phase is wrong.
Tor