What do you want to control about the on-screen display ? Do you want it not displayed at all, or at a different position ?
You can disable the OSD altogether in systemsettings > desktop behavior > workspace
uncheck 'display visual feedback for status changes'
You can also disable the OSD only for kmix, see the answer from
https://unix.stackexchange.com/question ... lasma-5-13 for a guide.
If you only want to supress showing the "Mute" event you would have to edit the plasmoid /usr/share/plasma/plasmoids/org.kde.plasma.volume/contents/ui/main.qml