I hit mute a couple of times. The mixer shows its unmuted, the menu for the tray icon shows its unmuted, but the on-screen display thinks its muted and no sound plays.
I restarted. The tray icon shows muted. I unmuted it, but there is no sound.
I then went to KDE and unmuted. This is a successful workaround.
Is there a way I can mute and unmute with XFCE?
My system is XFCE with Kubuntu 15.04, maybe an unusual combination. How to solve this?