KDE makes loud, startling noises

Open System Settings and click on Notifications:

In the Notifications window, first select the event source for which you want to configure notification, in this example, I chose Plasma Workspace. In the table, the first column reflects the state of an event. When a sound has been configured, a little play button is displayed. If you want to disable the sound for a certain event, click on the event and deselect the Play a sound checkbox. You can quickly disable all sounds for a certain event source by clicking the button Disable sounds for all of these events.

If you want to disable all notification sounds, you have to select each event source and click the "disable all" button. AFAIK, it's not possible to disable all sounds with one click.

However, it is possible to mute them. If you go again in System Settings -> Multimedia -> Audio Volume, you can simply mute all notification sounds:

Open /etc/pulse/daemon.conf as admin and set flat-volumes = no. Then restart pulseaudio with pulseaudio -k or logout and login. After that, system sounds should not max audio any longer.

PulseAudio, which is default for debian and a few friends, some 3-4 years ago made the decision to activate flat-volumes by default, which is stupid at best, downright assault at worst, and dangerous in any case.

I have to do a lot of work with fairly strong headphones. Therefore I often have the volume to 5% if I run music, as thats loud enough. Thanks to PulseAudio maxing the volume, now I (25yo) have Tinitus (permanently ringing ears).