A strange one, I'd log a bug but I'm not sure what component is the root cause here, and it happened in the last month or so... alas I was lazy/busy and a bad citizen to have not logged it before. Must try harder.

Anyway will try now. On latest KDE Neon User edition fully patched as of 2018-02-08

Package: plasma-desktop
Version: 4:5.12.0-0neon+16.04+xenial+build80

In Plasma, in the system tray, click the speaker, click on the extra "Audio volume settings..." expanded button over on the right which opens the new window. Click on Audio volume, you'll see the widgets are all corrupt, to prove they do work I can use krunner to open "Audio Vol..." and it opens fine, in fact I've linked screenshots of them, each of them has identical setting but one is mangled. It's really odd. It's the same for Device, Applications and Advanced but I've only screenshotted Devices...

https://i.imgur.com/uLCVnxW.png

https://i.imgur.com/i08a7qB.png

Well something fixed it in 5.12.1 so happy days...

https://www.kde.org/announcements/plasm ... ngelog.php