This was the problem I had.
I solved it (as described here ) by removing the analysis that Plex does overnight (loudness analysis)
Test it out by adding some new music (or deleting say one artist, updating and then restoring)
Just make sure your server settings are set to never analyse
I now can play all my plex songs by voice request on the Sonos One. (I had to remove and then add back all music so this did take a whiel for all the metadata etc to be retrived)
