From LQWiki

When working in an office, you do not want your computer beep. It is very annoying if for every tab completion you (and your colleagues) hear the system bell. Here is how to turn it off for different desktop environments:

KDE

• Test that your system beep is still there: open a konsole and type the tabulator key twice. Your computer beeps.
• Start kcontrol, choose "Regional & Accessibility" -> "Accessibility" -> "Bell". Un-tag "Use system bell" -> Apply
• Open another konsole and type the tabulator key twice. Now your computer does not beep.