Log out sound on GNU/Linux GDM on 4 easy steps

Open a terminal or Alt+F2 on Gnome (if you dont know what I’m talking about just STOP NOW!)
Write the following command: gksu gedit /etc/gdm/PostSession/Default
Before the last exit 0 add the following line:
/usr/bin/canberra-gtk-play --id="desktop-logout" --description="GNOME Logout"

If this do not work you can use the following line instead:

/usr/share/gnome/shutdown/libcanberra-logout-sound.sh
Save and next time you’ll logout it should work.