I've noticed this since 10.12.5 - for some reason, the volume of system sound effects keeps setting down to zero. I raise it up, but it ends up turned down again, after a while. Not sure when and what causes it. Note that this applies only to system sounds (the Alert sounds), not the output volume (I usually notice this when I erase a file or empty the Trash and don't hear the sound). I tried resetting SMC and NVRAM to no avail. I'm running Sierra 10.12.5 as I said, on MacBook Pro Touch Bar 15". Anyone else has this issue? --- Post Merged, May 22, 2017 --- Also: I noticed that when I turn off the option to display the volume controls in the menu bar, it turns back on after restart.