So the concept of stereo is VERY well supported now, but here are all the places this can go wrong:
Input Source --> Software playback --> Sound drivers --> OS or driver settings --> Sound card on Motherboard --> Headphone Jack --> Headphone Speakers --> Ears
I know glancing at this list, you probably have an idea where it's going wrong... "It's the drivers!" you might say, but you can't assume that any one of these is the culprit until you check them.
GUESS WHAT. I had this problem too. I literally just opened my computer with a screwdriver and realized that my motherboard cable internally had gotten loose to the headphone jack. That's right, the internal cable. It was hard to tell by looking at it, but I pushed that in tighter and it fixed the problem for me. (be sure you are grounded when doing this!!!)
Two months ago I saw the cables go bad inside the headphones themselves, and I literally cut the cable in half and replaced the cable by splicing once to fix it. No joke. I think the cable had gotten a car door slammed on it, and the wires got crimped.
But yeah, if you think about it physically there is just two streams that need to go through this pipeline to your ears, and it can break at any point, just check each one and you'll fix it eventually.