I have MacBook Pro 17" late 2011. After Mountain Lion installation, internal speakers are not working. I can hear soud from speaker during system boot, not n ot in OS. Speaker symbol is grey, not black. Headphones are working fine.
Re-installation, NVRAM clearing, etc. haven't solve the problem. I have discussed with Apple support, but they cannot to help.
I have also tried with OSX Lion, with that one speakers are working perfectly. So it cannot be a hardware problem.