Are you not getting sound directly from the jacks on the moterboard, or is it missing from the front panel connectors? Please post the type of case (there are two kinds of audio headers; one auto-selects, the other, I believe, doesn't...
Are you using XP SP 2? if yes try updating to SP 3. Had the same problem and a technician told me about that. CAn also try loading windows audio driver 1st then the realtek HD audio driver. Yes load them separately.
I have a friend who have a Gigabyte board (not this one) who also had the problem of no sound. I downloaded and installed the latest audio driver for the board and still doesn't have sound. After scratching my head for an hour or so, I tried using the driver CD that came with the board and WOOLAA! The sound is working!
You should trying using your driver CD to install the audio driver if you haven't done so yet and see if it works for you.
There have been numerous instances of one jack being eiter dead, or noisy, on some of the 'd' series boards. (If I recall correctly, it's the grey jack - but I couldn't swear to it...) This is easily fixed. Change your speaker plugs around, and write down which color you've got whatever speaker pair plugged into - then go into the audio manager, and right-click on the corresponding jacks on the upper right-hand side; select "connector retasking", and you will be able to set that jack to whatever speaker pair you have plugged in there... Be aware that 5.1 designates the rear speakers as 'side' speakers - with 5.1 selected, setting any jack to 'rear' will effectively kill it!