@Tpugliese - it's not in System Settings, it's in dconf. You have to:
1. Ensure you have unity-greeter 0.2.8-0ubuntu1 installed
2. Creating a file called: /usr/share/glib-2.0/schemas/50_unity-greeter.gschema.override
3. Place in this file:
play-ready-sound = false
4. Run this command:
$ sudo glib-compile-schemas /usr/share/glib-2.0/schemas/
Log out and no sound should be played.