Comment 38 for bug 437429

This has annoyed me greatly. After muting and lowering the Alert Volume to the lowest level, and changing /desktop/gnome/sound/event_sound to false, the drum sound still played.
Rather than figure out why none of these were working, I decided to just edit the file itself.

In /usr/share/sounds/ubuntu/stereo/ there are two files that concern this:
 - dialog-question.ogg
 - system-ready.ogg -> dialog-question.ogg

After renaming the symlink file with
sudo mv system-ready.ogg NOsystem-ready.ogg
the sound did not play!

While this is a really bad and temporary solution, it was the only thing I could do to remove the sound until a better solution came along.