s-video is only 4 pins, to do color. Audio is seperate. To get S-video out of the computer you need the s-video adapter or is you want you could custom make a cable to go from the computer to S-video pin out but cheaper to just buy the adapter and then you gain the digital audio output at the same time.
Watch out, the Dell dongle is not always compatible between different models. Get the one intended for your machine.