Wot Mike says re AC '97. Also, if you're using the front panel connectors, the pinout on the header socket for AC '97 differs from the pinout for Realtek HD (same pins, different order). So check you're using the right header connector on the cable to the headphone jacks. Also that front panel type is set correctly in BIOS.
Some Realtek audio chipsets aren't intuitive: if you're using two-channel audio or AC'97, you connect to line in and out rather than mic in and speaker out.
Personally, I'd just download the Realtek drivers. The Realtek Audio Manager is useful.