Hi,
I've noticed that the NVidia HD Audio drivers are just getting installed when you choose the manual installation and not the quick one.
Go to Chasitity's Thread:
http://www.asusrog.com/forums/showthread.php?1150-G73-Series-Driver-and-Application-ReferenceDownload Driver Sweeper 3.1.0
Download Nvidia Notebook Drivers 280.26 WHQL (08/10/11)
Then try this way:
Use the official uninstaller(s) of the driver(s) you want to uninstall.
Reboot your PC in Safe Mode.
Run Driver Sweeper and select what to clean.
Restart in normal mode.
Run the NVidia installation.
Choose manual installation.
Check all options and choose new/reinstallation.
Restart.
By The Way:
What BIOS update did you use? I didn't see any Update for the G74SX...