See if you have something that will plug into your speaker system and test that to rule it out. And, of course you're using the green jacks and have tried both front and back. I don't see your imageshack pictures well but I do see a digital option. You'll want to make sure that you're not outputting the wrong signal. Anyway, try to test the speakers by connecting some digital music player if you have one and see if that works.
So I have an HP Desktop.