First, try
this. If those drivers don't work...
If the automatic detection thing didn't work and the model doesn't show up on Sony's web site, they've seriously screwed up. There's a good chance the only drivers for sound that'll work will be from Sony itself, but let's see if we can figure out what motherboard you have.
Get CPU-Z from my signature and see if it will tell you what motherboard you have. At the very least, it should tell you what chipset you have. Post both or whichever it shows you.