Comment 1 for bug 364012

Probably a duplicate of: https://bugs.launchpad.net/ubuntu/+bug/214370

I worked around by adding a line like the following in /etc/gdm/PostSession/Default (though that seems kind of "dirty" to me)
/usr/bin/canberra-gtk-play --file=/usr/share/sounds/ubuntu/stereo/desktop-logout.ogg