Using the sound system panel, you can easily adjust settings to the sound output KDE and its applications produce.
KDE generates various warning signals, called beeps, when you make a mistake (e.g. copying a file into a folder you do not have access to). You can adjust the settings for these beeps here:
Sets the volume of the beep. You can set it from 0% (no beep) to 100% (you should think carefully before making a mistake after setting this...)
Adjusts the frequency of the tone. Ranges go from 0 (no beep) up to 2000 (you should not set this unless you have no pets like dogs or bats.)
Tells KDE how long the tone is played.