/usr/share/accountsservice/interfaces/com.ubuntu.AccountsService.Sound.xml is in accountsservice-ubuntu-schemas 0.0.7+17.10.20170922-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/accountsservice/interfaces/../../dbus-1/interfaces/com.ubuntu.AccountsService.Sound.xml