As much as I love KDE/Plasma, I have to say it, lately things seems to be getting more and more complicated, overloaded and less flexible, even messy at times. Anyway.
I dislike system sounds, it's a personal thing, and one that used to be very easy to configure. Since the latest KDE/Plasma, I've been meaning to get around to disabling audio notification. And when I say, "getting around to", what I mean is that there's no simple solution, especially not one that I can explain to an end user (aside from sticking a screwdriver into the jack port).
I've spent hours searching, on forums, various websites, IRC and even on Youtube videos (that's exactly how desperate I got). And finally, I took the time this sort of task now requires and I found 2 methods that work, I hope this helps someone save some time...
Brute Force Method
When there's no visible user clickable option, after going through all the menus, looking for search terms and so on, you can always remove the audio files. It works, a bit radical perhaps, and I wouldn't do this to a friend's computer, but it works.. It goes like this, as root:
mv /usr/share/sounds/ /usr/share/no_sounds/
No reboot required.
A New Hope
There's another way, and quite frankly, I don't know why this isn't the default setup. The short-long version is that the default audio backend is Phonon VLC and Phonon VLC does not allow audio volume for Event Sounds to be set separately (there's a bit of absurdity here). So, part of a usable solution is to change the back-end to Gstreamer
There's another little detail, you'll need to install Kmix to get to the audio channel for notifications so you can mute that.
First step, install stuff
apt install install phonon4qt5-backend-gstreamer kmix
Then, open systemsettings5 and go to Multimedia -> Audio and Video -> Backend and make Phonon Gstreamer the default.
And then, open Kmix and set the volume to 0, or mute the sound.
This should work, though you might need to reboot (or at least logout, I can't recall)... It seems this will not really stop the system from actually playing sounds, it will just play them at a muted (or lower) volume....
KDE/Plasma, we love you, but please, keep things simple and usable. By Default. Why isn't there a one-click easy-to-find solution to just turn off audio notifications ?