Comment 11 for bug 343677

I did a bit more testing. Sounds are played when both Net/EnableEventSounds and Net/EnableInputFeedbackSounds are set to 1, but not otherwise. This lines up with the descriptions of gtk-enable-event-sounds ("Whether to play any event sounds at all") and gtk-enable-input-feedback-sounds ("Whether to play event sounds as feedback to user input") from the GtkSettings documentation at http://library.gnome.org/devel/gtk/unstable/GtkSettings.html.