I bought Logitch Z506 speakers and when I try to set up 5.1 sound on my C-Media 8738-LX sound card (same thing happens with the motherboard) When just the green cable is plugged in my sub it outputs sound to the rear speakers but when I just plug in either the black or the orange cable it stops sending sound to the rear.
After you connect all the speakers to the sub correctlythen connect your phone to each input and make sure that the correct speakers work. That will eliminate a defect in the speakers.
The PC audio outputs color code is
Green for L-R front
Orange for Center - Sub
Black for L-R Rears
Set your audio to 5.1 in the audio control driver of the PC. If test tones are available you can check that each channel is working.