Audio problems with surround sound speakers

[Ninjaxor]

I have a Dell GX240 computer.
Running Windows XP Pro SP3.
I'm using a USB multi-channel audio device to hook up 7.1 audio to my home theater receiver.

I recently noticed that there is no sound in the rear speakers when watching 5.1 surround sound movies that uses an AC3 audio codec. I tried an AC3 5.1 channel audio test file that tests each channel, and it confirms that the rear speakers were not working. I tested this in two media players. VLC, and Windows Media Classic with the same results. I'm pretty sure I was getting sound out of the rear speakers before.

Next I tested a 5.1 surround sound DVD in my DVD player(separate from the computer) which resulted in all speakers working normally.

I then happened upon a speaker test in the software utility that came with my USB sound card. I ran the test and to my surprise the rear speakers were saying "front left," and "front right..." okay... that was really strange. I found a 5.1 and 7.1 test files on Microsoft's website that I tried with that also played the front speaker sounds out of the rear speakers

I've been searching on the web, and on this forum for help but have not found any similar problems. I'm greatful in advance for any ideas or help given

Thanks

Jon