If it’s actually a mDNS/multicast issue, you can run an mDNS proxy or similar to bridge the networks. I don’t have a Sonos setup to test with, but it works for other mDNS services.
My easy fix for that was to use Tasker to disable my Wireguard tunnel when on my home network and enable it again when I'm not. The official Wireguard app has Tasker integration, so you can use it to manage individual tunnels if you like.
