Crutchfield seems to say it has no eARC, but says nothing about ARC either way. How do you gave it connected to the Beam? 

Oops, I’m wrong, it does have ARC, just lower down in the specs. But the question stands. How do you have it connected? Is autoplay turned on on the ARC connection for the Beam?

Thank you very much for taking your time to answer my question.

Yes, I conected the videobeam from the arc output  with an HDMI cable to the Bean Gen 2.

So, that confirms one question (and your initial statement). Can you explain what you mean when you say ‘switch sources of sound at the Beamer’, please?

In general, when on Autoplay, the Beam will play any valid format signal it receives across the ARC on the HDMI connection. The assumption I’m taking is whatever you mean by ‘switch sources of sound’ means that you’re changing formats to something unrecognized. 

Have you called Sonos Support to discuss it?

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Thanks again for your answer. What I mean by changing sources is the following: I have usually two different sound sources, one is music I play from the Sonos App or the other source is the sound that comes from the video beam which in turn is the Cable TV source. In both cases the sources are valid and they’ve worked fine. What is not so evident is when I stop listening to the music and I want to integrate the sound of the videobeam across the Beam Gen2 to all the Sonos system

If you wish the TV source to continue playing to all the rooms that were playing the music source, you need to go into the settings for the Beam, scroll down to Home Theater, and switch ‘Ungroup on Autoplay” to ‘Off’.

Note that autoplay is triggered by the start of a signal. If the Sonos device is already receiving data across the connection, it won’t ‘switch’ on its own. You’d have to stop the video feed (and audio) first, if you want autoplay to function. If you don’t, ​@jgatie ‘s suggestion is valid. 

Tks Gentlemen

​@MLR 

But be aware that using tv mode audio with group option there is much more data traffic than using music mode. Depending on your wifi quality (if not having some or all devices being lan cabled) there may be drops of tv audio ob the grouped rooms. 
To avoid that you can use „audio delay in groups“ in Beams home theater settings. That increases the used buffer and reduces/ avoids drops, but the grouped rooms will play with a remarkable delay (what imho isn‘t a problem at all, if the grouped speakers are placed in a physical different room).