sorry to dig this old thread up..
but has this problem been solved?
I have regular SD not Infill version. Windows Media Player gives me all 4 speaker outputs, but SD only gives me the front 2.
Sometimes I get it to work by closing SD. Running WMP and starting a song, it plays in 4 speakers I then have to run SD and play a song on top of it and if it is in 4 speakers then i can exit WMP.
As you can tell this is a little annoying, plus it doesnt work ALL the time.