At the Asrock website for your motherboard the Audio driver download is 95MB!
Here's what it shows:
Windows® 7 64bit 95.09MB Realtek / VIA Audio Driver
Realtek sound drivers are only about a 35MB size.
So apparently it's using a a hybrid driver of Realtek and VIA. Go
HERE to download correct driver.
Also it's showing a BIOS update of 12-25-09. Looks like there has been 4 BIOS updates since Windows 7 was released.
It might be a good idea to do a BIOS update, if you're comfortable with it.
I hope all this will help you out!.