I've been trying to solve this for months now. I've updated ATI's Catalyst Control Center with every update they have released since March. I've uninstalled all RealTek drivers, removed the registry entries, restarted, and reinstalled fresh drivers from the RealTek site. I've installed all Vista updates and the problem persists. Here is my setup:
Gigabyte MA-GA78MA-SH2 Mobo
Sharp Aquos 46" LCD
Windows Vista Ultimate 64
Antec Fusion Case (not that this matters)
Basically, audio works fine via HDMI until I either turn the TV off or change the inputs on the TV. I know this is a problem with the TV not sending the EEID signal back to the HTPC but there has to be a simple software/driver fix for this. I'm about to "office space" this thing...
Someone help me before I do...