I have just managed to try out something which I thought might provide a proof of concept (or not) while we await WiiM’s DLNA Cast. I first set up LMS to enable it to look for UPnP servers (as described in the immediate previous posts above) and then added Shairtunes as an LMS plugin. When I opened Amazon Music on my iPhone, piCorePlayer became an AirPlay option to cast to. After connecting to it I was then able to use LMS to AirPlay cast synchronised music to multiple devices, in my case a WiiM Mini and a Sonos soundbar (see picture).
View attachment 15713
It means that currently I am listening to music through the following pathway:
Amazon Music on iPhone > Shairtunes LMS plugin (presumably AirPlay 1) > piCorePlayer (wirelessly) > LMS AirPlay > multiple AirPlay receiving devices (presumably using AirPlay 2).
My understanding is that Shairtunes effectively DLNA casts to suitable devices, so it seems just possible that if/when WiiM’s DLNA Cast feature becomes available then Raspberry Pi LMS users may be able to use this to also AirPlay cast to multiple devices in synchrony. Of course, this will depend on piCorePlayer being able to be ‘seen’.