Let’s hope there is a solution which we can share soon.
I had the same issue with a new Sonos Ray and a BT speaker. I just got it resolved today after calling Sonos Support. The issue was that the new speaker had a really old software version and for some reason the Sonos Mobile App was not able to get the software updated. The Support team said the only way to do the update is to use a desktop app. You can download the relevant one from here: https://support.sonos.com/en-gb/downloads
Once the desktop app was downloaded, I got it to join the existing system and then updated the software on the Sonos Ray. Once that was done I was able to play music from the desktop app.
The next thing was to sort out the Sonos Mobile app on my phone as it was still trying to update the software. I went to the Account Menu -> App preferences -> Reset App. I restarted the app and then selected Join Existing system.
It's all working as expected now. Really unfortunate that I had to call Support to get this sorted. If you try the instructions above and it doesn't work, just call support.