Correct me if I'm wrong but SB Live! doesn't support Dolby Digital Encoding so you'll only get 5.1 (digitally) if you have an AC3 track and using passthru to the SPDIF connector. In generel I really think that you should ditch the Creative's drivers for kX-Projects ones. The control panel gives you a much better view of what's going on and I've experiences less bugs compared to Creative's ones.
http://kxproject.lugosoft.com/index.php?skip=1
//Danne