I just spent hours, after discovering the music library capability has returned to the app, re-adding my music library on a synology NAS back to sonos. The whole process was hindered by a lack of proper instructions or error messages in the app. Let me save folks a lot of pain and give them the details:

1. Go into your NAS and make sure SMB is enabled. Also, make sure SMB 1 is turned off and 2 and/or 3 are on. I found that even if 2-3 were on, if SMB 1 was still possible it mucked things up. 
2. Go into advanced SMB and see if you can enable master browsing. This seemed to help sonos find the files I have music in later.
3. Do not try to add your music library from the app on an iphone or ipad. It is too hard, and does not let you browse to the files. You need to key them in by hand, which is really irritating.
4. Go into your PC (or MAC and do it there). Go into the add music library function. Use the browse function to find the music directory on the NAS you want to add. Note, if SMB 1 is active (and maybe if master browse is off) you may not be able to open your NAS folder under network. After multiple attempts (and making above changes) I succeeded in browsing to the correct folder.
5. Despite the fact that I did not put a password on the shared folders, it would not work unless I added the ID and password for the NAS.
6. This whole process took me something like two hours to work through, but it is now finally working and shows up on all the Apps. There needs to be a lot more documentation in the apps on this process. Also, if there is an SMB hiccup, the app should tell you instead of constantly redirecting you to fix your ID and password