help can't unmute in my Mac OS X Lion 10.7.2 (11C74)

System Preference > Sound

At the bottom there is a box after Output volume slider tagged Mute.

Uncheck the box.

If that does not help Reset SMC.

http://support.apple.com/kb/HT3964

Choose the method for:

"Resetting SMC on portables with a battery you should not remove on your own".

Best.