FWIW, here’s a tip for those whose Sonos app (iOS) does not see their music library.
This has been the case for me since the original app overhaul. My very large music library is stored on the 
drive of my desktop PC. The workaround for me is to stream something — anything — from the app and then the “Your Sources” header suddenly reappears (where previously only “Your Services” was visible) and my library with it.
It’s galling to have to do it this way but better than nothing. I realise that it might be because my PC could still be using SMBv1. However, the app is such a dumpster-fire that for the time being I am going to stick with this workaround rather than experiment further.
I hope this helps anyone who has had problems getting the app to recognise their stored media.