I’ve had this problem for a while and it got fixed yesterday by a Sonos support rep.
“Unable to add the music folder”, “Error 913”, “music files not available”, “username or password incorrect”, I’ve had them all and tried every solution I could find in the articles - multiple times. The short version of the fix that worked was that the support rep made a new folder on the Mac under “users” and copied all my music to it. Then he went to Sonos/ Manage Library Settings, clicked +, selected “Networked Folder and made the path “/users/”the new folder name” and it worked. I was stunned as I had no expectation this would solve the problem.
I fully expect a new version or something will break this again in the future but for now I can play all the music on my hard drive in iTunes/Apple Music. Hope this helps somebody out there.
