Re:680I SLI MB Rev D
Make sure onboard audio is enabled in the BIOS.
On other thing that may very well be happening is Nvidia now also has a audio driver that gets installed by default when you install their graphics driver, that may be conflicting with the realtek driver. You can uninstall the Nvidia audio driver from add/remove programs I believe.
The latest realtek audio driver can be had here www.realtek.com,
go to quick links on the right choose HD audio then on the actual download page choose the 32 or 64 bit version for win 7, if you do not know if you have 32 or 64 bit you can download the 5th listed item that has both the 32 and 64 bit in the same package.
EVGA x58 E758 BIOS 83, i7 920 C0 @ 3.3GHZ, Corsair 750TX, EVGA GTX 480 HC FTW 11th anniversary prize THX EVGA, OCZ3P1600LV6GK, OCZ Vertex 2 SSD 120GB, Logitech X-540 5.1, LG W2453V, Swiftech H20-220-Ultra-GT WC kit with XSPC rx360, Windows 7 pro 64 bit OEM