First of all, are you using a coaxial or optical (TOSlink) SPDIF-cable? And there should be a program in your controlpanel called Soundmanager or something like that (blue icon, if I remember correctly) if it's Realtek. There you should be able to choose what sort of cable connection setup you're using. I had the same problem with my Asus P5N-T Deluxe mobo (also 780i), but it turned out to be a faulty SPDIF-connector on the board (it was bent). After RMA and getting a new one, everything came automagically when I booted the PC for the first time. Anyway, check all of your sound-settings, then double check. If you can't find anything, then post back and well see if anyone else has a better answer to your problem.