Hi,
I just want to *LINK ANOTHER THREAD* where we are discussing this.
It may be helpful to others...
_2 known quick fixes :_
1. go to sound preference select another sound source...then go back to internal speakers
2. unplug and replug the usb cord connected to your portable mac (Macbook, Macbook air, macbook pro)
_2. possible solutions :_
1. get a better UPS power supply (maybe a power source problem)
2. simplify connections to the back of the display... especially hubs plugged into the cinema display usb hub. make sure all hubs are powered (if being used)
*LINK (please read) :*
http://discussions.apple.com/thread.jspa?messageID=10473201#10473201