$ sudo chown -R gdm:gdm /var/lib/gdm
[sudo] password for vd: 

$ sudo -u gdm gsettings set org.gnome.desktop.sound event-sounds false

** (process:28766): WARNING **: Command line `dbus-launch --autolaunch=b6e7b648e27de6d3e62e67ac00000009 --binary-syntax --close-stderr' exited with non-zero exit status 1: No protocol specified\nNo protocol specified\nAutolaunch error: X11 initialization failed.\n

** (process:28766): WARNING **: Command line `dbus-launch --autolaunch=b6e7b648e27de6d3e62e67ac00000009 --binary-syntax --close-stderr' exited with non-zero exit status 1: No protocol specified\nNo protocol specified\nAutolaunch error: X11 initialization failed.\n