/usr/share/gnome-control-center/icons/hicolor/48x48/devices/audio-speaker-mono.svg is in gnome-control-center-data 1:3.28.1-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/gnome-control-center/icons/hicolor/48x48/devices/audio-speaker-center.svg