Double check the sources area in the new app. Sometimes the new buggy app just won’t show it. Force close the app a few times or uninstall and reinstall. 
 

If not luck are you using smbv1 or http shares? These are no longer supported. You would need to change whatever you use to host your files to smbv2/3 and re add the share with the desktop app. 
 

If you aren’t sure how to do this there are posts on the community or consult your device support

Everything that was suggested has already been done. Unfortunately, the "music library" feature is simply no longer integrated.
The software developers have never heard of "user requirements".
The most important (and only) thing for a good user-friendly interface.