Are you sure that's the right model number? I know that the 81945 bit is wrong; it's 8I945, but the rest may also be wrong at that rate. I looked around gigabyte's website for you pretty generally and found nothing related that number.
If it is correct, then i'm guessing that it's some special, customized board like those used by PC manufacturers such as Dell. These are notorious for having wierdo model-numbers, and drivers sometimes never become fully available. If you have such a machine, you're probably best off rooting out the discs that originally came with it and installing it from there.
If, on the other hand you can find out what the sound chipset is, you can go straight to their website (for example Realtek or Soundmax). You could grab some diagnostic software like Lavalys' Everest, Belarc or Sisoft's Sandra. This will give you precise info. After which, googe is your friend
