Master volume control is grayed out
I ran into issues where my audio controls were disabled similar to what you describe. I can't be sure it's the same issue, but in my case, killing the Core Audio process fixed it.

Open up Terminal and enter this command:

sudo kill -9 `ps ax | grep 'coreaudio[a-z]' | awk '{print $1}'`


You'll be prompted to enter your password, but that should find and kill the proper process, which will then auto-restart. Your volume control should come back to normal within a few seconds if this is indeed the problem.