Sound not playing through speakers, but playing through headphones?

I've posted this issue before on numerous support fourms, and none of them or I could figure out this issue. It's been driving me nuts.

A couple months ago I cleaned my computer (plugged everything out, cleaned the area/computer, etc.) when I put everything back together and booted up, I had no sound, but the tray icon indicated sound was indeed going through. As far as I knew nothing had been changed. I made sure everything was plugged in correctly, reinstalled the drivers, made sure I didn't click something off I shouldn't have, but still nothing. But then I plugged in headphones into the tower headphone jack, and boom I had sound. I was as surprised as everyone else. Someone suggested then, to rule out a hardware issue, that I download Ubuntu and see if the sound problem occurs there. It didn't, I had sound coming out of my speakers.

I would hate to re-install Win, as I've gone without doing so for five years. But if it came down to it, it's whatever.

I use the on-board sound from my MB, as I've learned over the years sound cards are a thing of the past. But perhaps I could reattach the old one I have? It's sadly over 10 years old however, I don't know if it would be worth trying. Any help would be greatly appreciated. Major cudos if we're able to figure this out once and for all. Good luck

My system:

Windows 7 Ultimate 64-Bit
AMD Phenom II X4 965
Gigabyte GA-870A-UD3 (on-board sound)
Ripjaws 8 GB
Nvida Geforce GTX 780

Hopefully these are the correct ones you were asking for. Let me know if it isn't!

Sorry for the late reply. I actually checked the back and it seems I mistakenly put the green plug into the grey (which I swear looks like an off green) and that was the issue I had. I knew it was something as simple as that, but jeez. One of those things you have to bang your head against the wall for.

Thank you for suggesting that, otherwise this would be going on for a lot longer than needed :)