I know Sonos integration is awesome but also is beta and stops working within the ST system a lot of times. Over on Twitter I was given advice that has kept my Sonos working for months without needing to remove / re-add the speakers into the system: Reserve IPs for all Sonos speakers.
Doing this one simple thing makes ALL the difference. I didn’t do my bridge device, only the speakers. For some reason once they’re on reserved IPs and then added into ST, they continue to work. The IP address of the speakers remaining the same is key to making it work. I hope this helps people with their Sonos & SmartThings setup.