Comment 12 for bug 258703

Looks like there's a few parts to this one:

1. To enable all the controls on the gnome-sound-properties page, the freedesktop sound theme (from comment 7) needs to be installed to /usr/share/sounds
2. The instructions in this mailing list post (https://tango.0pointer.de/pipermail/libcanberra-discuss/2008-July/000021.html) need to be followed to enable libcanberra gtk integration. Quoted here:

"...Packagers!

If you package libcanberra, make sure to include a script like this
one in your package:

<snip>
#!/bin/sh

if [ -z "$GTK_MODULES" ] ; then
        GTK_MODULES="libcanberra-gtk-module"
else
        GTK_MODULES="$GTK_MODULES:libcanberra-gtk-module"
fi

export GTK_MODULES
</snip>

This script should be sourced in each Gnome session and will enable
the libcanberra gtk module for all gtk programs.

In fedora we install it as "libcanberra-gtk-module.sh" in
/etc/X11/xinit/xinitrc.d/. I assume other distributions have similar
directories.
..."

I think this second part is missing from the ubuntu libcanberra packages, which is preventing the system sounds from working properly even when the freedesktop theme is installed.