Hey did you ever figure out a fix for this? I have the exact same problem using Audition CS5.5, Asus Xonar Essence STX. It's hooked up to a home theatre using SPDIF. I can ping the individual speakers, and the surround works great in games, but in for audition, no matter where I move the thing, it only uses the two speakers.
Tried ASIO and MME in Audio Hardware, no diff. Mapping options are not making the surround work. This is the one glitch in an otherwise perfect editing system.
Could someone please help me with this? Would love to see Adobe and Asus bang heads for a solution here. Obviously I'm not the only one.
Many many many thanks,
Throwing out a wild guess here, but maybe DD encoding on the fly just won't work. Try using the analog out of the sound card.
What do you have the Xonar settings set to in the windows control panel as far as the digital out and the sample rate?