Catalyst v12.4 drivers contain a bug related to HDMI connection. When the display is turned off, the sound is no longer working and in device manager, the sound device is disconnected until you restart your PC.
The only way to fix this, is to remove and re-insert the HDMI cable. Another solution is to disable "AMD External Events Utility" from Services (go to Start -> Run and enter "services.msc").
You can download the beta drivers, as many users are saying that the problem is fixed.
Update: The solutions above are not solving the problem. The bug has to do when Windows are checking if the display is turned off. A temporary solution is to disconnect Pin 19 from your HDMI cable: