Ok i was wondering if u guys could help me, as im sure im going crazy for a simple mistake.....

I have a 5.1 logitech system, i forget what model, but i have a 10' sub.

Anyways, i hook up the sub, center, front left and rights as soon as i got it. Everything sounds amazing. Then i decided about 2 months later, to hook up the left and right rears.
I run wire through the attic, make some splices, drill holes in walls ect....

After all this hard work, i hook them up and the rears sound horrible. They sound very "tin can like" and cut in and out. I tried reversing wires thinking i had just hooked something up backwards, but to luck. I tried fiddling with the computer settings...looks like a realtek driver? not sure but thats what pops up when i go to configure the speakers and test each one. What the heck am i doing wrong here? Or maybe its just that i can't listen to music with surround sound on? do i have to keep it in stereo?

any help would be great