At least in my Debians, e.g. 11, 12, 13 (14.1.4~pre) on host g5eas, all of them online upgrades, the tray popup for "Mixer" has no mute button above the slider. Instead, there are 3 tiny triangles above the slider that do nothing when clicked. Right ckicking the speaker icon and then toggling mute has no effect either. I've made no changes to the applet's defaults.