I used to have a Sound Blaster until I switched over to Win 7 and it wasn't supported ! Which is why I now have an Asus Xonar D1. As I said, I am only an occasional gamer and am willing to settle for standard stereo sound - it works fine for radio and CDs which is what I listen to mainly. But, looking at the control panel for my Xonar there is a wide range of output options, and I imagine your Sound Blaster has the same options ( at least ).
Audio channel - set to 4
Sample rate - set to PCM 96kHz ( this is the default and it seems to work fine )
Analogue out - set to 4 ( choices include 2, 4, 5.1, 7.1, FP headphone, FP 2 speakers )
Before you can say for certain that you have a real problem, you need to make sure that the soundtrack you are playing has rear-speaker sound.
You say that your front speakers are working fine, and that your rear speakers are also working when you plug them into the front speaker socket. As a test, have you tried plugging your front speakers into the rear speaker socket ? Remember you have three line out sockets on that sound card - make sure you are using the correct ones.
If all else fails, have a look at Creative's FAQs and forums, assuming they have one and perhaps contact their Help, or your supplier. It's not impossible that you have a faulty card, or some other problem with your computer may be preventing it working properly. You could also try re-seating the card.