/usr/lib/x86_64-linux-gnu/libmad.so is in libmad0-dev 0.15.1b-8+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmad.so.0.2.1
/usr/lib/x86_64-linux-gnu/libmad.so.0.2.1