To enable Creative's 5.1 stereo expansion (CMSS), open up the Creative PlayCenter, make sure you're in "player view" not in "Jukebox view", click on the CMSS button (located underneath the number pad on the application window), enable CMSS. You can now close PlayCenter or leave it open. In any case, you'll now get 5.1 expansion from every sound source (mp3, CDs, line in, .wav, etc.).
Note that CMSS has never sounded and will never sound as good as QMSS (Philips Acoustic Edge) or Vitual 5.1 (Turtle Beach Santa Cruz) or even "virtual 5.1" found on the Hercules sound cards (GTXP). Try it out and you'll see what I'm taking about. Creative wanted to create a 5.1 expansion scheme in which the surrounds act as full reverb and the center channel gets priority focus... good intensions but it still sucks. The Audigy 2 will feature a newer version of CMSS which is supposedly much improved over the CMSS found on the Audigy and Live! 5.1 family sound cards.
Yes, you will actually get 5.1 sound (without enabling CMSS) on certain games using DS3D or EAX (not all!!). Games that utitlize the center channel are Max Payne, Unreal Tournament, Unreal Tournament 2003, I think Return to Castle Wolfenstein, and several others.
Get a hold of a softDVD player capable of decoding the DD bitstream. That's when you'll appreciate a 5.1 speaker system most.