I have had this issue, on and off. Usually, I'd have to reboot the computer entirely. However, for myself anyway, I found a much easier fix! If/When the volume becomes stuck, if you have a wireless keyboard, as I do (Logitech K800, for me!), simply turn
the power off and wait a few seconds. The volume bar on screen will disappear.
Then turn it back on, and the volume should stay at the level you put it at. I think it has something to do with some flaw using the keyboard volume buttons. When doing it manually via the little sound horn in the lower right-hand corner or via my wireless
headphones, I never have the problem. Only when using the keyboard controls.
Good luck and I hope this helps someone!