You spin the wheel on the keyboard, and the overlay appears on screen to let you know the volume has changed. Or you pop open the menu and adjust it there. Everything looks good. But the volume doesn't change.
Because sometimes, the sound is coming out of the speakers, but the volume control is somehow attached to the monitor port on my microphone. So when you're adjusting the volume, you're adjusting the volume of that.
This is a corporate build of Ubuntu 18, so it's pretty ancient. If i boot into current Fedora - same hardware, same desktop environment - it works fine.