To turn off the annoying KDE login/logout jingles (which doesn't respect your sound settings and plays loudly when you are at the library, at a lecture, a conference, next to someone asleep, etc, unless you remembered to have headphones plugged in):
Go to the KDE system settings program, click "Notifications" icon, "System notifications", "Applications" tab, select "KDE Systems notification" in dropdown menu, then select "login", then uncheck the "Play a sound" checkbox.
Done. Whew. They hid that pretty well.