how are they connected? and have you tried another program such as foobar2000? that comes with a simple stereo > 4 channel plugin which is enough to check rears are working.
The rears work with the latest drivers, now it's just a tinny mushy mess.
also one other thing to check is how windows is actually configured. right click on the speaker in the system tray >> playback devices >> select your speakers or whatever output you are using, click on configure and run through the wizzard
5.1 sound, it all works perfectly from a windows point of view, click each speaker and get a nice loud non-tinny ringing sound through each and every one of them, the issues start when I attempt to use winamp (latest version).
I'm going to install a different player now that I have proper 5.1 with the latest drivers to see if it's a winamp issue, which I suspect it now is.