check the mixer - you may have to turn the volume up on the rear speakers..
I know it sounds a bit silly, but I've turned them down when using the CMSS surround in 5.1
on the Creative X-Fi, and then wondered what was wrong when I've switched back from 2.1 stereo..
you should be able to get the 5.1 configuration, so keep trying to correct that one,
rather than switching to 7.1
PS. the plug colours should match, but make sure that you have connected the speakers correctly
- refer to the manual, if you have one