Since the upgrade on the Sonia app I cannot find my music library. 

Using. Mac? Same for me. This works. It didn’t at first but I hadn’t entered the full path name in Step 3

100% credit to the OP

https://en.community.sonos.com/controllers-and-music-services-229131/how-to-solve-the-error-913-music-library-on-a-mac-6894161?postid=16732714#post16732714

H thank you!  I was able to solve the problem after waiting 80 minutes for a live person on the phone for Sonos customer support. (while walking my dog- so that worked! By the time I got home they got on the line. They screen shares with me  and essentially did the necessary tweaks -now that Sonos read updated the app. I wished they would tell people this!  Thank you for your reply. Now I have it documented if I need to do it again