oh there is another tab on AC3 filter where you can check use SPDIF, its the tab up at the top in the settings window that says SPDIF, on that page there is a box you can check that say use SPDIF try that.
also is the HDMI cable your attempting to get sound from hooked to the video card?
if this still fails what you can do is if you have multi-channel outputs on your sound card is just hook them directly to the receiver, if not and it is only stereo what you can do is hook it up via that and set AC3 filter to matrix DD 5.1/ DTS etc, into dolby pro logic II, then select that setting on your receiver when you want to watch a movie and you will get fairly accurate surround sound.
if you cant figure out how to do this tell me and ill take a screenshot