Asus do the xonar Dx2 range that has digital, however why are you pumping the sound back into your PC? What PC speakers do you have?
you can use the multi AV cable that came with the PS3 and use the RCA jacks Red /White to connect to a 3.5mm jack on a set of PC speakers...
Oh, Asus do have a HDMI card that you can actually use to accept the HDMI signal and this will handle both video and audio but a bit pricey for a bit of console gaming, unless you want DTS media playback, but might as well use your PC direct for that..
I know some games have DTS support etc... Depends on how much want to have feature wise and want to spend.