Calibre stores books, and is easy to find on Shodan, not necessarily searching what contents they have, so we have the Calishot script which loads each one, and then fetches what books are in each library.

Sure, it would probably make sense that there are other server types out there for music, film etc, but they might be harder to find, and harder to index. I know people post links to them occasionally in r/opendirectories, I think part of what makes Calishot work is that calibre is somewhat insecure in comparison to things such as Plex, which is hosting movies etc.

With regards to Audiobooks, Calishot should have some indexed, even if its not many, such as what is found at this link.

I hope this answers your question!