Yeah this definitely SOUNDS like a Motherboard issue, however, I have some questions to make sure...
1) Do you have lots of cables right next to each other, in a straight line? You could be getting cross talking. Even if you unplugged your speaker from the end of a long cable connected to your computer, it's possible it could still be sending the signal along the wire.
2) Do you have any software utilities like mixers installed? It could be that you installed a digital mixer on your computer and you set it up to allow you to output your voice with some overlayed audio on the computer.
3) Have you tried reinstalling pulseaudio?
4) Have you tried deleting pulseaudio's config file?
5) When did this issue start to happen?
I hope I was helpful