VIA HD Audio Not Working w/ 5.1 Channel Speakers

I got a new computer, and it appears the integrated sound card does 100% well on my 5.1 speakers. The new motherboard's integrated soundcard has 6 audio jack-ports in the back (the typical multi-colored jacks for front/back/center/side speakers). Problem is that my speaker system only has 1 audio jack for it to go out.

Now when I test each individual speaker, it only works for the front left/right speakers. Center/Back left+right do not work.

However when I play my music (and set my speaker to "Surround" mode, sound comes out of all speakers, but more of the volume comes from the front left/right speakers.

I've tried fiddling around in the sound settings int he Control Panel to find no solution. I was wondering if anyone had a similar problem to this or not and if they managed to fix it.

I've thought about getting an optical cable and connecting it to the speaker sytem and to the computer, but I don't know if that'd help.

Specs:
-Asus M4A785-M Motherboard (ASUSTeK Computer Inc. - Motherboards- ASUS M4A785-M)
-VIA VT1708S High Definition Audio 8-Channel Sound card
-Windows 7 64-bit
-Durabrand HT-395 Home Theater System (can't find a URL to this sytem since its pretty old)

This has nothing to do with Windows or the onboard audio, and everything to do with what and how you are connecting too. Found the manual for it here,

Durabrand HT395 Home Theater User Manual

If you are connecting to it using only a single cable from the computer to the HT, then you need to set it to stereo in the Windows settings as that is all that will be output. You then need to use the HT to upmix the stereo signal to surround.

Another option would be to use an optical cable from the motherboard to the HT and run it that way. At least using the optical connection anything that is native 5.1 will/should play back in 5.1 on the HT.