For me this started about a month ago and I found a quick fix. Go to terminal and enter:

killall mate-panel

This kills the mate-panel. Then, the panel will automatically come back, and some stuff that was broken may now be less broken.

This worked to restore the volume control until about two weeks ago, and that stopped working. I probably changed something else on the panel. So, the above fix may or may not work for you.

A second fix, not as good, is working for me. I opened the Control Center, and found the “sound” section (under “Hardware”) Then, I dragged that to the panel.

This allows you access to the volume control, though not as a simple widget. Rather, it give you the whole sound system configuration dialog.

Once I did that I realized that I’ll just keep that on the panel even when the sound applet starts working again, because it is pretty darn convenient.

Meanwhile, yes, this is a bug, it has been reported and it is being discussed. I think this is simply something that will eventually be fixed, and after some future upgrade you’ll find it is no longer a problem.