/usr/lib/x86_64-linux-gnu/libdmx.so is in libdmx-dev 1:1.1.3-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libdmx.so.1.0.0
/usr/lib/x86_64-linux-gnu/libdmx.so.1.0.0