I don't know if you managed to fix this, but I had the exact same problem and after checking everything I found the solution, so I'm answering for future reference:
1)Go to the Creative Volume Panel
2)Go to the CMSS panel
3)Click the surround button
4)Check that Surround is selected (not stereo xpand)
5)Go back to the CMSS panel and move the stereo envelopment slider to the middle (or to the percentage you desire)
If that doen't work at first, right click on the windows sound icon on your task bar. Select playback devices, select your x-fi soundcard and click on configure. Make sure 5.1 speakers are selected, then click next twice. I found that sometimes disabling "full range speakers" works too. If full range speakers are selected, be sure to tick the surround box too.
I don't know if it's a problem with the drivers or Windows 7 itself, but sometimes the settings will change all by themselves. It has happened to me a few times since I installed Windows 7.
I recently upgraded to Windows 7 from XP, I have 5.1 speakers however my rear speakers will not work when I listen to music, other speakers work fine. I have an X-Fi Xtreme Music soundcard and I got the Win 7 driver from Creative's site (installed using Vista compatibility). All speakers worked great in XP and I have uninstalled/reinstalled the driver a few times.
Everything in the Creative Console and Control Panel show I have 5.1 speakers and when I test them I can hear noise out of all of them ("rear left, rear right, etc..."). I have also made sure to enable CMSS and the volume is up. One thing that is kinda strange is that when I enable EAX effects and select an effect (such as jazz club) the sound comes out of the rear speakers. I am using Windows Media player and to make sure that wasn't the problem I went online to watch a video and was still missing sound out of the rear speakers.
I understand both Win 7 and the driver for my soundcard are beta but i'm just wondering if anyone has any ideas on how I could fix this problem or if i'm pretty much stuck until a new driver gets released?
Any help would be greatly appreciated, thank you very much.