I,ve got exact the same problem, and it’s not the first time. It happened before at 7 version also. Now I’ve got 8.0b and it’s happening again.
I launched which ctrl+shift but it only deleted my cache and set default settings – sound is still unavailable.
I’m at the end of almost 2 month project and all version of my project even those from last week suddenly got the same problem, so it really looks like some sudden bug in Vegas – not in projects.
Only thing that is sorting this out ( in some way ) is saving as Edit Protocol Compliant AAF. After reopening sound is there but it looses effects settings, markers and so on 😐