In most cases like this, you must connect the board's audio solution to the graphics card using a S/PDIF cable. The (3-pin) S/PDIF header on the Dragontail Peak (DP35DP) board is present in the schematics and is labelled as J1BU. Unfortunately, when I checked the TPS document for this board, I found no mention of this header at all. This may mean that, while the location exists for this header on the board, the (3-pin) connector may not be stuffed. All I can suggest is that you look at your board and see if you can find this header. If it has the (3-pin) connector stuffed, you are good to go and can get a S/PDIF cable and connect it up. If it is not stuffed, however, you likely are out of luck.
Note, the S/PDIF output from the audio codec is routed to the S/PDIF header location and then on to the Optical S/PDIF output on the back panel's audio jack block. The S/PDIF header location should thus be somewhere very close to - typically right behind - the back panel's audio jack block.
If the header is not populated, you have a couple of paths of recourse - but only if you (or someone you know) is into electronics. First of all, if you are very good with a soldering iron, you could attempt to stuff this connector yourself. Alternatively, you could put something together that converts the optical S/PDIF output signal (available on the back panel's audio jack block) back into the simple S/PDIF signal that is needed by the graphics card. I don't know how to do this but I imagine an electronics buff could accomplish it...
Hope this helps,