Apple has today released an update to fix the problems some users were experiencing with their 27-inch LED Cinema displays. The update is not a standard one so certain manual operations need to be carried out.
After the download the updater application is installed in the Utilities folder and should launch automatically, you then need to follow the instructions within the application to correct the problems if you are experiencing them.
Previous support for Apple stated a reconnect fix for the problem:
“You may notice that sound from an LED Cinema Display (27-inch) connected to a Mac intermittently becomes inaudible. In some circumstances, you may need to unplug the display and reconnect it or restart the Mac before the sound becomes audible again.”