I've been using Yosemite since the first DP was released. Never had this problem before.
Things I have tried till now:
Repairing permissions fixed the problem once for me, but the volume icon greyed out again a few hours later.
Things I have tried till now:
- Resetting PRAM
- Verifying and Repairing Permissions
- Killing coreaudiod using the terminal
Repairing permissions fixed the problem once for me, but the volume icon greyed out again a few hours later.