Some speakers aren't working

So, here’s the situation: I have a 5.1 speaker setup, and even though my sound settings are configured for 5.1 surround, I’m only getting audio out of two channels. To make matters worse, the subwoofer is completely unresponsive—no matter how much I crank the bass control on the unit itself, nothing happens. Just to be clear, everything worked perfectly fine on my old PC, so this isn't an issue with the speakers themselves. It's definitely something with the new rig.

control panel -> sound properties: just tell him he’s got a 5.1 speaker setup...

I gave it another shot, and still nothing works.

It’s likely an issue with your audio drivers or maybe just the sound card itself.
Does the audio setup on your new rig actually support 5.1 surround sound?

Sound card High Definition Audio Controller [1002-AA58] [NoDB]

Sound card VIA VT1828S @ Intel Ibex peak PCH - High Definition Audio Controller [B-2]

This is what I managed to dig up regarding the sound card in Everest.

I'm running a 5.1 speaker setup myself, but I've only got two working right now. Because of that, I definitely need a new sound card that actually supports 5.1 surround sound—and honestly, I think you're in the same boat.
As for that lack of bass, just try installing the audio driver.
If you’ve already got it installed and things still aren't sounding right, try a clean reinstall.

I was just playing CoD5 and all my speakers were working fine, but now when I try to listen to music, only two are working. So, it clearly isn't an issue with the sound card itself.

That’s just because you're listening to music recorded in stereo, so your other speakers don't really have anything to do. I suppose there might be a way to fix that, but I'm not entirely sure how. If you were to do a little digging on Google, you'd probably find something.