/usr/lib/x86_64-linux-gnu/libout123.so is in libmpg123-dev 1.23.8-1+b1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libout123.so.0.1.4
/usr/lib/x86_64-linux-gnu/libout123.so.0.1.4