Spekaer help please

Hey, i bought a set of Z 640 5.1 speakers (logitech) yesterday, and when i unhooked my old ones and hooked these up theres no sound from the rear speakers >< and yes my settings are set to 5.1 in sounds and audio devices. i also dont get ANY sound from them even when watching DVD's in WMP, heres my specs hope you guys can help

Asus A7N8X Deluxe Motherboard (using the onboard 5.1 sound)
Radeon X800 pro 256MB
1 gig of ram (not sure of brand)
ADM Athlon XP 2500+ cpu

sorry for the double post but wanted to get a mod or admins attention to let them know they can close this thread or delete it if they would like i solved my problem...

I haven't worked with the z-640's in particular...I know that for the z-680's you need an actual pci sound card that is 5 or 7 channels....this very well be the case...I would just go to logitechs site they should be able to help you out dude...good luck with the awesome speaks.

Also check your balance and fade levels both on your pc, and on the external remote thingy...