I run Xfce and a few KDE apps, but I haven't the full KDE desktop installed. When the KDE apps show notifications, some KDE subsystem library plays a sound (I assume it's knotify but I'm not sure) and it raises the sound volume.
There used to be a systemsettings command that let users access sound settings also when not running KDE, but that's not the case anymore, since the new systemsettings5 does not offer audio configuration.
A workaround is removing read permissions to the sound files with
    # chmod o-r /usr/share/sounds/KDE*
but that obviously falls short of a real solution.

Reproducible: Always

Steps to Reproduce:
1. Install Xfce and Konquest, but not the full KDE environment
2. Turn sound volume off in Xfce
3. Play konquest and look for a way to silence it

Actual Results:  
Konquest plays a sound on each notification and there's no applet or command to adjust the sound volume for it (or any other KDE app),

Expected Results:  
KDE apps sould respect the sound volume set by the user in a different desktop environment or KDE libraries should offer a command to adjust their sound volume (the latter being what I suggest here).