How do I turn the feedback sound ("pop sound") of my touch bar volume buttons back on?

Dear all,

Very suddenly, the volume buttons on my touch bar have stopped making that popping sound when I turn the volume up or down. As you can see from the screenshot, both the play user interface sound effects and the play feedback when volume is changed options are enabled. Still, no feedback sounds when I change the volume.

In fact, earlier today, my MBP didn't even play any sounds suddenly - not in the browser or itunes. I then did the control, command, r, p button procedure and the sound is back on. I have in-coming call sounds, music etc has sound; it's just the sound when adjusting the volume that is missing still.

By the way, I have a MBP 15 inch, 2016 with touch bar, and macOS Mojave 10.14.4. Everything should be up to date.

Thanks!!