I'm not sure whether it should go to generic-wayland or specificly to System Tray Component, so I'm writing to the former. I've just noticed that a System Tray does not load Audio Volume module for me. When the module is gone, the OSD that should indicate volume changes/mute does not appear on the screen. The change itself also does not seem to happen. With Audio Volume KCM I can not see any changes feedback when manipulating volume rockers. The "Audio Volume (automatic load)" in the system tray settings is of course present and checked. I was able to add the Audio Volume widget to the panel. In result, everything works as expected. Volume changes, mute works, OSD is displayed and the KCM shows changes that are happening. Plasma 5.10.1 Qt 5.9.0
*** This bug has been marked as a duplicate of bug 381010 ***