Did you let windows update the driver? This happened to me once. I had to uninstall the driver in device manager and then install the correct driver. Installing over the old driver never worked. Download the newest driver from Gigabyte, then unistall the driver in device manager and then install the new downloaded driver. Hope this works for ya!
Have the driver downloaded and ready to click on to install, go to device manager and find the sound device and right click on the Realtek High Def Audio driver and select uninstall. Once it is uninstalled the sound device should dissapear from device manager. Now install the downloaded driver and restart your comp. If this does not work I do not know what else to tell ya. It should work.