MBA to Cinema Display sound

[Blarnld]

I'm on a 2012 MBA using 10.8.5 (just updated last week) .My 27" LED Cinema Display has recently gone quiet. In system preferences I have the option for Sound/output selected on LED Cinema Display. No sound! When I select Display Audio, I get sound. Looking online there was a problem like this happening a year or so ago. An Apple SW Cinema Display update 1.1 was said to fix this problem. I tried to download this update, but the keep getting the message that my computer does not need this update. I assume that my computer already has update 1.1 yet to problem exists. Unplugging Display and restarting computer solves this problem until the computer sleeps, then on waking, the problem is back. Any help?