MacBook Pro 15" 2011 - missing audio device (no sound driver)
I've resolved this before (grey icon, no sound) by resetting the PRAM.
- Turn off your Mac
- Turn on your Mac
- Hold Down Cmd-Opt-P-R
- Keep Holding until you have heard the startup sound twice
Apple Support Document HT1379
If resetting the PRAM doesn't work, you may try installing the latest Combo Update, like this one:
- OS X Yosemite 10.10.5 Update - https://support.apple.com/kb/DL1833?locale=en_US