It is not easy to find, topeira@kde-brasil told me: Go to System Settings, Notifications, Player Settings, select “No audio output”. Click Apply, it’s done. 
Disable sound notifications on KDE 4
07
May
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It is not easy to find, topeira@kde-brasil told me: Go to System Settings, Notifications, Player Settings, select “No audio output”. Click Apply, it’s done.
Pier
May 27, 2008 at 5:08 am
Great! You helped me to remove that annoying sound while changing desktop in the pager!
Pier
Tom
July 10, 2008 at 9:13 pm
thanks!
that system bell every time i opened a preferences dialogue was driving me nuts!