Way too many variables here. What player are you using, what sound card, is your computer set up as "4 speaker surround", is it a stereo or surround source ...
I'm betting it's just the nature of the speakers to replicate the low/midrange to the back in an attempt to create a better soundstage. Try unplugging the rear inputs from the computer and see if it does a full clone. Failing that, you'll need a 1/8" splitter to replicate front to rear and just switch plugs as needed, or a plugin for your media player to say "Okay, output an identical signal to front and rear"
Example now, using my 5.1 Logitech set - Winamp without a plugin only plays through the front + sub. WMP10 goes to all four, blended front to the center, and the sub.
- M4H