Download this and look at Page 1-22:http://dlsvr02.asus....494_p4v8x-x.pdf
(manual for your board)
Take the side off your case and see where Number 9 is, front panel audio. Do you have anything plugged onto those connectors or do you just have two jumpers either side of the missing pin?
If you have nothing at all on this set of pins, turn your computer off and find two small jumpers from somewhere (off old hard drives or CD units for example) and use them on either side of the pin that is (deliberately by design) missing.
On the other hand, if you have eight or so little connectors on this jumper block, they are probably wired up incorrectly, so a quick solution is to turn off the computer, pull them all out, stick two jumpers on the appropriate pins (see above) and then you should have sound.