Music Library Empty, only the Folder path Shows in app

Have you called Sonos Support directly to discuss it?

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

As a guess, it has to do with some wonky sharing settings, as explained mostly in this thread, but there simply isn’t enough information in your post to be sure. Which is why calling Sonos support may be helpful. 

It sounds frustrating, especially since it was working fine before. First, make sure the external drive is properly indexed by macOS. Try re-indexing the drive by adding it to and then removing it from the Spotlight privacy settings.

If that doesn’t work, you might want to try refreshing the music library in the app or even resetting it to force a rescan of the folder. Also, ensure the app has full disk access permissions in your Mac's settings.

If the issue persists, it might be worth testing with a different music app to see if the problem is specific to the software you’re using.

Hope this helps!

Thanks- I've not had a chance to call, and wasn't sure how easy that was (help lines not always being a positive experience!) but I may need to find time. I was able to solve the 913 error through other people's experiences and comments around the whole sharing issue, but I guess it might be easier to just get Sonos on it directly now!