Ensure the PlexAmp App, has been given access to the LAN subnet that both your mobile device and Plex server/Sonos devices are using - sign in with your Plex account and your Sonos products should appear in its cast list … see screenshot from my Home setup:

Thanks for the reply. I have both my android phone and sonos speakers connected to the same WiFi from my router on my home network so can't see why they aren't on the same subnet already. Anyway I'll continue to tinker .

Thanks again

I think I had the same issue a while back where I could only see Chromecast devices and not my Sonos devices within Plex and PlexAmp.
I removed and re-added/authenticated the Plex service from within Sonos and this fixed it for me.
Might be worth a try?

Ross.

@Rod25, I would try a reboot of the local router to see if that fixes the ‘discovery’ issue and maybe sign out/in on the PlexAmp App… if there are any other WiFi access points/extenders (that’s not including WiFi mesh Hubs) on the local network, then power off those too, just to ensure all are using the router WiFi AP only.