Grant wants to only hear his headphone sound in mono because he's only got good hearing in one ear. Leo says that PopOS has only Surround and Stereo. But there's bound to be a mono sound driver online somewhere. Probably the easiest way would be to get a hardware option. People used to be able to get one at RadioShack. But sadly, that's in the past. Doctor Mom says that he can make headphones mono with an adapter from Monoprice. Here's how - https://www.cnet.com/news/making-headphones-mono/.