/usr/lib/x86_64-linux-gnu/libfluidsynth.so is in libfluidsynth-dev 1.1.6-3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libfluidsynth.so.1
/usr/lib/x86_64-linux-gnu/libfluidsynth.so.1