Okay, after uninstalling and reinstalling drivers, after swapping out sound cards (onboard to external), after changing cables and all, I found out my Vizio soundbar that I use as my speaker was overheating.
Not sure there is a solution for it. I tried blowing canned air into its vents. But it just may be that I've worn the poor thing out.
Anyway, it was the speaker and not the motherboard.