Center/Sub audio signal not working on Logitech Z-640

[WaitWhat4355]

I have a 5.1 Logitech Z-640 audio system that used to be hooked up to a Desktop we have that no one used and sat unplugged, and when I got my own desktop, I decided to claim it as mine since no one used it. All 5 speakers and sub still work, but there is an issue with audio that goes to the Center/Sub speakers. I know it is not the wire, but something on the center speak which has the audio controls. When trying to test my audio with the windows Speaker Setup, audio comes through the FrontLeft, FrontRight, RearLeft, and RearRight speakers, but nothing comes out of the Sub and Center. When listening to videos and games while the computer is in 5.1 or quadraphonic, the base still makes sound, so I'm going to assume it outputs the low sound signals that are for the 4 working speakers. While fiddling with the dials on the center speaker and testing to see if any of them are the problem, I found that the one labeled "center", which I'm assuming is for making the sound centered, more to the left, or right, appears to be the issue. I believe this because when using Speaker Setup and making it test Center and Sub while I move the dial, the speakers crackle a bit and you can hear the test sound, sometimes I am able to get a decently loud, but I'm sure not as loud as it should be, sound from them, but when watching videos or playing games, you can hear the audio cut in and out from the center speaker. Am I correct to assume that the dial is a variable resister that is going bad? Or would it be something else? If it is a bad VR, I can find a replacement one easily and swap it myself; the hard part will be opening the control panel on it, I think.

 

[BBrent]

I believe its you're sound card or lack of sound card. If your computer can't output 5.1 it's only going to output a stereo signal to the 2 main speakers.

 

[WaitWhat4355]

I believe its you're sound card or lack of sound card. If your computer can't output 5.1 it's only going to output a stereo signal to the 2 main speakers.

It is not my computer. It can support 7.1 surround sound. As I said before, near the end of my first post, when I move the dial labeled "center" while there is supposed to be audio coming through the center speaker, it crackles and audio comes through. If I have the patience, I can get it to were it produces decently loud, but not as loud as it should be, sound, but goes in and out when I play a game or audio. When I just move the dial and try not to get it to make sound, it will most likely not produce anything. I believe this is a bad Variable Resister, but I'm not quite sure. And I know it is not my desktop, I can swap the Center/Sub plug with, say, my RearLeft/RearRight plug, and when I use the Speaker Setup test, the Center and Sub audio outputs on my rear speakers, and when it goes to test the rear, the Center and Sub don't make a sound, unless fiddling with the dial.

*EDIT*
Just found out the dial labeled "center" is a volume control for the center/sub speakers, so it would make sense that it is a bad Variable Resister, as putting it to the max does not fix it.