/usr/share/glib-2.0/schemas/10_gnome-session-canberra.gschema.override is in gnome-session-canberra 0.28-3ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | [org.gnome.desktop.sound]
theme-name = 'ubuntu'
|