I really don't like the sounds which Kubuntu plays during startup, or during thinks like moving a file to Trash. How can I disable all such notification sounds?
Some applications and subsystems of your workspace can notify you about various events. Here you can specify which events you wish to be notified about, and how you want to be notified.
System Settings > Application and System Notifications > Manage Notifications.
Pick the application. The KDE Workspace has the trash, logout, login etc notifications.