Two thoughts.

First, the ‘place’ where the speaker is getting the signal is different in those two situations, as is, potentially, the frequency being used. When set up as a ‘standard’ room, the speaker is using either SonosNet, if you’ve got a device wired to your router, or hanging off your WiFi. In most cases, that’s a 2.4Ghz signal, created near or on your router.

When set up as surrounds, the speakers are on a ‘unseen’ 5Ghz network connected to your Sonos Beam. Since the source, and potentially frequency is fifteenth, you can be getting some sort of wifi interference in one setup, and not the other.

I would recommend reviewing the linked FAQ for any potential fixes that might be applied to your situation.

Second, unless you either post or call in with the diagnostic number, it’s unlikely to be looked at in a sea of ‘oops, I didn’t mean to do that’ false submissions. See the Diagnostics - How do they work? thread.

Thanks. 
that helps as I hadn’t realised the surround set up worked over a 5Ghz network. 
It will be interference from my wifi mesh node and/or Sky Q and the linked mini boxes. 
gives me something to work on. Cheers.