Tic Tac Toe
Have you installed the drivers from the CD in the drivers menu, select 'realtek ALC850 Driver'
You also have to enable the on board sound in the bios. You a re looking for 'AC97 Audio' in the Advanced menu this should be set to 'Enabled'.
[The manual says 'connect one end of the front panel audio I/O module cable to this connector.']
I believe this refers to the extension sockets for the mic and headphone sockets on the front of the case.