Basically what happens is that you are able to look at your media files up to the point of playing them than you get an error that the file play the file as it is unavailable. I removed and added the service back and now it can’t even find any files.
What I’m after is finding out how Sonos communicates with Plex and if I need any special ports are required.
Best answer by MikeV
Browsing your music library really requires the Remote Access functionality of Plex to be enabled. Sonos communicates with Plex's own servers, which retrieve the needed library data from your Plex server and format it to be displayed on Sonos.
Playing music technically still needs the Remote Access function, but it relies on a router function called "Hairpin NAT" to keep your internet connection from being used. When you go to play music, Plex provides Sonos with your router's public IP address as the source. For routers that support Hairpin NAT, either internally or through an advanced setting, the router will realize that the connection from your Sonos speaker(s) is intended for your Plex server, and will route that connection back to the Plex server without ever touching the internet.
While many people have their Plex server on the same network as Sonos, Plex's use of the public IP address also allows you to access a remote Plex server to play music from it as well, like a remote family member's server if they're using the Plex Home functionality of Plex Pass.
Nothing more is needed than the port forward that Plex requires for regular remote access, so that should be easy.