Repair disc permissions is always a good idea. I also saw someone report this on the official thread. They said re starting restored their audio, but if you had the issue in RC 3, I am not sure it would help you.
Have you tried booting from your install disc and running NBI from there? I have had success doing that, as you would in Safe Mode.
Mini 9 | OS X 10.6.7 | NBI 20100616212351| BIOS 05 | 32 GB Runcore | 2 GIGS RAM
10V | OS X 10.6.7 | NBI 20100616212351 |BIOS 06|160 GB|1 GIG RAM| 6 Cell Dell Battery
Back up Mini 9| OS X 10.6.7| NBI 20100616212351| BIOS 05 | 32 GB SuperTalent | 2 GIGS RAM