For example, this fork of mopidy-soundcloud: https://github.com/ticosax/mopidy-soundcloud
The docs at https://docs.mopidy.com/en/latest/installation/debian/#installing-extensions don’t really cover manual install when Mopidy itself was installed with apt-get.
Even the extension installation instructions down in the installed-from-source Mopidy section on that page just reference pip.
There’s a bunch of forks of the completely-broken Soundcloud extension, but I’m not really sure how to properly install any of them.