[CentOS] Sound problem, possibly Mate Volume Control

Hi all!

In the last day or two (on Centos 7)the speaker icon in the Mate top panel, which normally drops down to a volume control, has stopped working.

what does "stopped working" mean?

It means when I drop down the volume slider and slide it, it makes no difference at all in the output volume.

if I right-click that icon, it'll open up "Sound Preferences", which appears to be the same as "Mate Volume Control". Within that program, the input or output level slider DOES work.

I fired up alsactrl and it seems to work fine, and will also control volume.

I dont have pavucontrol (or is it pavuctl) installed, so I don't suppose it has gotten messed up.

Any clues?

thanks!