/usr/bin/musixflx is in texlive-music 2015.20160320-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/bin/../share/texlive/texmf-dist/scripts/musixtex/musixflx.lua
/usr/bin/../share/texlive/texmf-dist/scripts/musixtex/musixflx.lua