I’ve found that adding my iTunes library (instead of trying to add the actual external path) imports all my playlists from Music into the Sonos app. But the actual music still isn’t there.

I have already given ‘full disk access’ to both the Sonos app and SonosLibraryServer.

Okay, wait a minute. I’ve solved this, I think.

I undid everything above and started from scratch.

1. First I go to my Security settings and give ‘full disk access’ to the Sonos App.
2. Then I go to the Sonos App on my Mac Mini.
3. I go to Music Library settings.
4. I add the folder. Path shows as /Volumes/4TB drive/Music
5. Music Library appears in my list of sources, but contains no music.

Now I go back to my Security settings and SonosLibraryServer has appeared in the list of apps, so I can give full disk access to that as well.

1. Back to the Sonos app. Remove my folder.
2. Add my folder/path again.
3. This time it takes a long time to read and set up my library.

And it works. All my music shows up in the Sonos app, on all my devices.

No playlists though, of course. So this still isn’t perfect …

Further progress, in case it helps anyone else:

1. I went back to the Sonos app on my Mac.
2. Added a second folder - the local Music/iTunes folder (which in my case holds no music, just the Music Library.musiclibrary database file.

And now I have a successful Imported Playlists section on Sonos. It worked!

So for those of us Mac users with Music collections on external drives, the solution is to add both folders to Sonos - the actual music files location and the system standard Music folder.