disable ready (drum) sound in ubuntu 12.04

Create (or edit) the following file:
/usr/share/glib-2.0/schemas/50_unity-greeter.gschema.override

Add to it the following section and content:
[com.canonical.unity-greeter]
play-ready-sound = false

Make the OS aware of the changes:
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/