I have this motherboard, and no additional sound card.
My case comes with both the HDA plug and the AC97. I used to have the HDA plugged in, but I got no sound out of my front headphone jacks. Also tried AC97, with the same result.
Under Windows Device Manager, I can see the Realtek High Definition Audio Device listed. I have also tried the jacks at the back of the computer, with no success. However, whenever I play sound there is a little green bar that pops up and down to indicate volume, and seems to suggest that the sound is playing...somewhere. I don't have speakers, and they sure aren't coming through my headphone jacks.
I also have a Radeon 5750 which I think has HDMI audio if that changes anything.
I'm fairly sure the wires are plugged in right, and I have the newest drivers. Am I missing a setting somewhere?
Realtek ALC889 is HD, so use the front panel audio HD connector.
Did you try connecting your headphones to the green audio jack on the motherboard (rear panel)? If you got sound doing that, then your front panel audio cable may be damaged, or you did not properly connect the FP audio to the motherboard's audio pins.