Windows 7: 5.1 Two Rear Speakers Not Working

Please help me with my problem. My PC configuration looks like this.
Mobo : Intel DX58SO
CPU : Intel i7 950
I have a 5.1 Creative LX520 Speaker, I have set the sound output to 5.1 in control panel, but when I test the speakers the two rear speakers does not work. But if I change the setting to 7.1 it works, but the sound from the rear speakers are like distorted. Please help.

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