Although I don't use KDE as a desktop, I use a few KDE applications from time to time, mostly kdiff3. Among other things, kdiff3's what Mercurial prefers to use when resolving conflicts in a 'hg pull -u', which comes up from time to time as I have a custom copy of the Firefox development tree. For a while now, kdiff3 and the occasional other KDE applications I use have been making noises at me to notify me of various things. I'm very strongly against programs making noises at me and normally turn this stuff off, but this time around I couldn't find an obvious way to do it in places like kdiff3's own application settings. Normal people might reach for their desktop's general settings, but for my sins I don't use a desktop environment; I use a custom setup built around fvwm as my window manager.

I was all set to write an unhappy entry lamenting how modern desktops really didn't believe in people who were doing this. Then I did a bit more Internet searching and managed to find some suggestions, so now I can tell you what seems to have worked so far for me:

• run 'systemsettings', which will get you the KDE 5 system settings.
• go to 'Audio', which is in the 'Hardware' section
• turn 'Notification Sounds' down to zero volume.

Contrary to what you might think, you won't find this setting in the 'Notifcations' section, which in any case is greyed out for me because KDE thinks I'm using the GNOME notification system (which I sort of am, because I run /usr/libexec/notification-daemon by hand in my session).

(My Fedora desktop has both a 'systemsettings' and a 'systemsettings5' program, but the latter is just a symlink to the former.)

I haven't logged out and back in again, so it's possible that this setting will reset itself when I do so. Or perhaps it's been persisted in some magic place, although I did some Internet searches for obvious (or readily findable) KDE configuration files, and none of them seemed to have this (and it's not in gsettings that I could spot).

I wish all of this was better documented, but it's a futile wish at this point. Linux desktops have limited resources, so of course they prioritize documentation for people actually using their desktop (or some other reasonably popular desktop, or at least an actual desktop with all of the expected pieces), not people running weird custom environments.