I expect this will resolve your issue.

Unwired surrounds often use a 5ghz ad-hoc network connection to the main player, so I would look to see if there is something interfering, either physically in the room itself, or perhaps other WiFi devices operating nearby on the same channel - a common culprit is a baby monitor for example.
 

Try simply rebooting the router and all the speakers in the Home Theatre setup and just see if that fixes it - the Sonos Home Theatre will often seek the best channel for its operation/connection to its surrounds.