The greeter options are somewhat unchangeable as a user though you can see them all in dconf-editor > com.canonical.unity-greeter
You could try disabling the sound there, whether it works or not don't know.
(in various bug reports on such it's been posted how to effect such changes, don't bother with anymore..
Another way that works here is to change the default, whether you should do so I don't know, did so here but I've no real attachment to my ubuntu install(s), ect.
scroll down to bottom & edit the default for "play-ready-sound" to false
gksudo gedit /usr/share/glib-2.0/schemas/com.canonical.unity-greeter.gschema.xml
Then save, close gedit
Should execute without comment or error, if it does read carefully
sudo glib-compile-schemas /usr/share/glib-2.0/schemas
Then try a restart & see..