Does anyone know how to get sound output through hdmi out in nvidia gts450 card and optical spdif output on mobo ant the same time?

I have just purchased the new gts450 from nvidia to replace a 9800gt with a dodgy fan issue, the 9800gt needed a spdif cable to connect to the mobo for sound passthrough to hdmi but the new nvidia cards don't need this cable and i do get sound through the hdmi out from the graphics card but only if the default sound device is the hdtv output.
If i make the default sound device the realteck audio one i get sound through the optical spdif out for my amp but i can't seem to have both at the same time.
Anyone having similar problms with the new graphics cards in htpc's