I can see this topic has been raised multiple times previously, so I'm surprised it has never been addressed by Sonos: for music stored on a phone, why is there no folder view? It is a simple programming fix included by multiple other music apps. The included Artist/Album/etc views are useless, given that the app is very sensitive to ID tagging differences and splits some albums into multiple albums (for instance if 1 song on an album is tagged as "pop" and another is tagged as "alternative", they appear as separate albums). Also, mix albums get split over multiple artists.
A folder view allows users who care about the organization of their music (a user base which highly correlates with users wanting high quality speakers like Sonos) to control their listening experience, which is the point of technology. This missing feature makes me regret having switched to the Sonos ecosystem (I have wasted hours on workarounds, including re-tagging my music, only for Sonos to find some other reason to split songs amongst albums).