Because the Roam can’t be used as a surround speaker, it must connect to your router, and not the Beam directly.

I suspect you have a level of wifi interference that is enough to disturb the bandwidth necessary for music, but not enough to disturb conversations when grouped with the Beam. 

Try moving the Roam temporarily, closer perhaps to your router for confirmation. Of course, read the linked FAQ.

However, you may want to submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. But don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

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 Airgetlam.

The Roam is playing in the same room as the Router, which is why I am puzzled. No more than 3m apart and nothing between them. The Router is up high as I mentioned.

I have used the Sonos support staff a few times and submitted diagnostic reports. I find them very competent and helpful. However I think that Sonos could provide more info arranged around workflow relating to common tasks or problems. The features-based user manuals may be great for marketing & sales but lacking the coherence required when dealing with a workflow task such as initial setups, grouping and operational issues like this. The articles about WiFi problems were of no help to me with this issue. All suugested solutions had been considered.

I appreciate your prompt response.