Sonos is (or was) mostly UPnP with some extensions... so for discovery they used SSDP https://en.wikipedia.org/wiki/Simple_Service_Discovery_Proto....

I messed around with their stack and internals for a while about ten years ago... some remnants remain

https://github.com/NathanHowell/Sonority/tree/master/

https://github.com/NathanHowell/Sonority/tree/core/

https://github.com/NathanHowell/upd2

They use SOAP on UDP. I played with it a while back using Sonex https://github.com/harmon25/sonex Have a fork to make some things work under my username.