you can try to use "sudo -u gdm dbus-launch gconftool-2 --set /apps/gdm/simple-greeter/settings-manager-plugins/sound/active --type bool false"
you can try to use "sudo -u gdm dbus-launch gconftool-2 --set /apps/gdm/ simple- greeter/ settings- manager- plugins/ sound/active --type bool false"