Assuming all is ruining in your WiFi, It might be the wireless connection perhaps, particularly if the ‘Move’ is say using the 5Ghz WiFi band and the Arc is on the 2.4Ghz band for music playback, whilst it communicates with its surrounds over an ad-hoc 5Ghz connection.

Maybe first try a reboot of the Home Theatre system and if that doesn’t improve things, then consider switching the ‘Move’ onto your routers 2.4Ghz band (if it’s not on that band already) ensure the routers WiFi is set to a fixed non-overlapping channel, 1, 6 or 11 and perhaps set the channel width to 20MHz to reduce interference and see if that then fixes things.

Thanks for the advice Ken.

How do I check what band the Move and Arc are on?

And presumably to switch the Move onto a new band I'd have to do that from my router settings? Not via the Sonos app?

The current network in use by Sonos is shown in the Sonos App “Settings/System/Network” area - it will show what network you have in use. The easiest way to switch the ‘Move’ to a different band is to factory reset it and set it up again with a Sonos controller device connected to the band you wish to use… or you can add another WiFi network to the ‘Move’ instead using the procedure shown below:

1. Connect your mobile controller device to the new WiFi network. (Perhaps ensure any previous network is not accessible by disabling its beacon temporarily) Select the 2.4Ghz/or 5Ghz band (as required for setup).
2. Open the Sonos App and wait whilst it searches for your current Sonos system.
3. When the app shows “Unable to connect to Sonos” ... tap the  ‘Learn More’ link
4. Tap the ‘Changed Router or WiFi Settings’ button and select the ‘1 product option’.
5. Follow the instructions to connect the Sonos Move to the new WiFi network. NB: The Move will connect to the same signal as that being used by the controller device.

Hi @JamesU2021 

Welcome to the Sonos Community!

Are you still having this problem? If so, please try recreating the group but with the Move selected first, then group the Arc room to it and play - is this better? If so, the Arc may have a link to your WiFi that is just good enough to get the stream, but not to send it as well. If this turns out to be the case, please look at our help page on reducing wireless interference. 

If thing’s still aren’t working, please get in touch with our technical support team, who have tools that will enable them to give you specific advice about your system.