Fixed! After seeing that no-one else seemed to be experiencing this issue, I finally resorted to a Reset All in the prefs and it solved the audio problem.
Odd that the old prefs in v2.8 did not show the same problem.
I was very much looking forward to the 3.0 update because v2.8 was causing certain movies of varied formats to freeze on startup. v3.0 seems to have fixed that issue but now I find that many movies in avi, mkv and mp4 formats suddenly have NO AUDIO AT ALL! ZERO SOUND. Their audio is fine on v2.8. I'...