Also check the ports in the back. I know that with the Beta versions of Vista i had to use the kX Project drivers which switched the ports on the back of my sound card. I had to manually plug the outgoing sound into my mic slot. I knwo it sounds wierd.
But turn some music on and see if switching the plug from one port to the next gets you some sound. If so then there is 1 of 2 things wrong.
1. Plug in wrong port to begin with. I have done it several thousand times.
2. The drivers are buggy and switched the ports like they did in the Beta versions of Vista.
Either way is seems there is something wrong. Best bet if you have installed them, espically more than once, you might be having a driver conflict. If you had drivers previously installed and then installed these use Driver Cleaner Pro. Found here...
http://downloads.guru3d.com/download.php?det=745
Install that and run it. Clean out all drivers that you know of. then try to install the drivers linked above again. If nothing then it seems there could still be a bad driver or driver conflict.