Comment 21 for bug 282906

This should not be "invalid", turning the "/desktop/gnome/peripherals/keyboard/bell_mode" off does nothing, nautilus/gnome will still play the "alert sound". Unfortunately, there seems to be no way to turn off the alert sound, only a way to choose one of the annoying alert sounds provided. The only "fix" I currently see is to disable all sounds by selecting "No sounds" as the gnome sound theme, which is not a good work-around.

I came across this as deleting files in nautilus now causes these alert sounds, very annoying as moving something to the trash is not an error.

Discussion on the subject:
http://ubuntuforums.org/showthread.php?p=3669007