Binary package hint: gnome-settings-daemon
First of all, a quick introduction to the (unfortunately not very well-known) component we have had for a few releases now, called libCanberra, which is used for sound events in the same style as icons.
In upstream's gnome-settings-daemon, the system tries to play the audio-volume-change sound effect when the user adjusts the volume with media control keys.
Granted, the sound is unimplemented in Ubuntu's default sound theme, but it absolutely exists in others. With the Freedesktop theme, the sound effect is an extremely helpful "pop" noise that indicates what the new volume is.
Although it only affects users with different sound themes, losing this feature is a regression. It also limits what can be done with Ubuntu's own sound theme, if that gets updated at some point.
Here is how the sound is played in gnome-settings-daemon ./plugins/media-keys/gsd-media-keys-manager.c in the update_dialog function:
ca_gtk_play_for_widget (manager->priv->dialog, 0,
CA_PROP_EVENT_ID, "audio-volume-change",
CA_PROP_EVENT_DESCRIPTION, "volume changed through key press",
CA_PROP_APPLICATION_ID, "org.gnome.VolumeControl",
NULL);
With our patch to gnome-settings-daemon to use libnotify for this event, the sound is not played, causing this rather unfortunate regression.
Please implement this feature for when a volume change is triggered in the libnotify-using version of gnome-settings-daemon.
update_dialog is still called with our patch, else the notification wouldn't work (the hooks for the libnotify notification are in update_dialog, so I'm not sure why you think it's never called)