A problem that seems to be affecting quite a few users after the Mountain Lion upgrade is loss of sound in iTunes, QuickTime and other apps. Some people are reporting that the Volume control is greyed out or won’t move.
The resolution here is to quit and resart the coreaudio process. To do so:
1. Open Activity Monitor.app (Applications > Utilities > Activity Monitor.app)
2. Select ‘All Processes’ from the drop down menu next to the search bar (called ‘Filter:’) and type in ‘coreaudio’.
3. Select the process name and click the ‘Quit Process’ button at the top. Chose ‘Quit’ or ‘Force Quit’ from the resulting dialogue box.
The coreaudio process will automatically restart itself (if you look closely at the PID number in Activity Monitor you’ll notice it changes after you hit ‘Quit’) and your sound problems should be solved. However, if you find that this procedure only partially solves the problem or doesn’t work for you, also try this:
4. Open Terminal.app (Applications/Utilities/Terminal.app) and copy/paste this command into the Terminal window
then press ‘return’ on your keyboard.
5. Restart your mac and test.
6. Still not working? Check your audio configuration here:
/Applications/Utilities/Audio MIDI Setup.app.
Make sure the window has the title Audio Devices, and click ‘Built in Output’ in the sidebar. Are any of the boxes under ‘Mute’ in the main panel checked? If so, uncheck them. Is the format set to 44100.0hz? If not, change it so that it is.
Hope this helps!