I have a rudimentary HTPC I built from an old P4 motherboard originally to stream Netflix to my HDTV and Dolby Digital enabled receiver. I was able to do well finding a Nvidia 6200 AGP video card with HDMI out but struggled with the sound card. I wanted on with an S/PDIF optical out to run to my receiver. S/PDIF is either the optical TOSLINK or Coaxial digital connection you commonly see.
I started with a TurtleBeach Riviera. I struggled with it for a few weeks until finally reading that it does not pass through Dolby Digital (DD) but instead down converts it to 2-channel 48hz PCM. My next try was a Diamond ExtremeSound but I read it too will not pass through DD so it is going back. What is the point of having DD output on a card that don't.
Belatedly, but happily, I discovered my retired ASUS PC4800-E Deluxe motherboard had an onboard coaxial S/PDIF out that actually will light up DD on my receiver.