I also posted for similar help in the ubiquiti forums but I guess this stuff is too old hat to most of them and didn't get a response.
For those in the know does mdns only casts between the 2 subnets (the one where the Sonos is located and the one with the controller app on Android or IOS) that we specify in the router config? Similarly does the same occur when we set up an IGMP proxy?
I'm not too keen on the Sonos broadcasting across all my subnets.