Looks like Amazon is planning to end your ability to upload or play personal music files, and many predict that all of the other music streaming services will eventually do the same. https://arstechnica.com/gadgets/2017/12/amazon-music-ends-mp3-upload-support-will-end-music-storage-service-in-2019/
This is a blow to folks who have collected tracks not offered by any music service and are unhappy with the reduced development of local library features and management. The answer for those needing to play files from their local library has always been "Just upload them to a streaming service", but now that option is disappearing with no good alternative being offered. Let's hope Sonos redirects some resources back to the local library side of the house before the ability to play those treasured files that don't exist in Music Service libraries disappears for good.