What you want is zmusic:app with the -w switch.

    -w zmusic:app --socket /tmp/uwsgi-zmusic.sock --chmod-socket=660

Using --chdir to change to the backend/ directory may be useful as well.

I've just updated the README with these instructions.

I've also added a section on using uwsgi in standalone mode, as the built-in debugging server can't handle concurrent requests.