What did Sonos support say, when you called in to them?

Called them?

I followed their troubleshooting options.

I messaged them on twitter.

I didn't realise there is a number I can ring them on. Could you share it please

When they tell me what the issue is, I'll come back and let others know for future reference 

Sure, here is a duplication of the link at the bottom of this page:

Sonos Support 

I can think of several things that could be going on, but there’s just not enough information to tie it down. 

My first guess would be an issue with the firmware on the TV not being up to date. Sonos is pretty current (and sure, worth checking the Sonos is up to date as well) on their implementation of CEC/ARC, but if there’s some sort of disconnect there, it might possibly cause issues. But in general, the HDMI cable is a pretty straight and well understood connection. 

But if the CEC system on the TV isn’t working properly, it might not be accepting the information from the Beam that says what it can play, and so your Fire TV and XBox could be sending invalid formats to the TV set, which is then passing it on to the Sonos, which wouldn’t be able to play invalid formats. 

Usually, that hard wire connection means that there’s less of a chance of wifi interference being an issue, but who knows? Sonos might be able to help there. 

I’d certainly be tempted to look at the settings for both devices to see if you can turn off CEC….getting one of those Lindy CEC-less adapters, while cheap, may not (or may) be necessary, depending on the devices and if that’s even the issue.