Can't add music library to Sonos Mac Mini

Usually a call to support will let them check your setup and offer a solution.

Other users that use Apple gear may offer a fix here when they drop in.

 Is the folder that contains your music listed in the file sharing page ( the “i” next to the File Sharing toggle )?  What error do you get from Sonos when you try to index your library?  Are you using the Sonos app to attempt the index? 

SonolibraryServer doesn’t need access if you’ve setup a standard network share under Mac OS. That indicates that you may have used one of the two options in the Sonos Mac App that no longer work, and that Sonos refuse to simply remove to stop causing confusion. Only the third option will work now. 

https://support.apple.com/en-gb/guide/mac-help/mh17131/mac

You need to follow the Apple instructions to make your music available at a folder level on the network and then use option 3 to connect Sonos to it with something like 

//mac-ip-address/sharename

Sonos instructions here, although I would not use the computer name and would use the IP address instead,