Well the drivers are installed right, the speakers are in the right port. Everything is set to High... Thats all i can think of... I'd start to think that your onboard sound has had it.
One thing you could do though, since truthfully onboard sound is not great for listening to music or playing games, even if it is a 6 speaker set up or something. It will just take up CPU time.
I can honestly say to you that it will be best if you bought a Creative Sound Blaster card. If you just want Stereo, you don't even have to get the latest Audigy ZX 2. I'm happy with my Sound Blaster Live card and supports 4.1 speakers. This would only cost about £20 now probs, and if you wanted more speakers, yuo could go for the latest which would set you back about £60, but you'd get excellent sound
They just plug into a spare PCI port and this is the best option. When you get it, you'll have to disable the onboard sound and then just install the drivers for this instead and i assure you that it'd work as soon as everythings connected.