remove lightdm sound

from https://bugs.launchpad.net/ubuntu/+source/unity-greeter/+bug/949782



Create a file called: /usr/share/glib-2.0/schemas/50_unity-greeter.gschema.override

with content

[com.canonical.unity-greeter]
play-ready-sound = false

Run
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/