It seems like there should be an icon in the rc menu for the sound applet:
What icons and theme do you use. If it's not stock please contact the dev of that theme or icon set
I created them https://ubuntu-mate.community/t/custom-installers-for-cursors-icons-and-emerald-themes/14914/10?u=terrowinIt doesn't matter if it's those, Adwaita, or Ambiant MATE, the icon is missing in the RC menu itself.
Edit: See, here's the Radiant-MATE icons (I logged out and back in first):
Here's some screenshots of how it should look:
This is now fixed upstream in mate-media 1.18.2.