Currently I am deciding which AV Receiver to buy, one from Yamaha with MusicCast or one from Sony with Songpal. It is important to me that the multiroom support works in the respective Home Assistant integration.
This thread is considering the Sony Songpal Mediaplayer platform (Sony Songpal - Home Assistant).
From my limited ability to understand the code, it seems that the media_player.join action (Media player - Home Assistant) is not supported by the songpal media_player platform, although this functionality is present in the underlying python-songpal library (GitHub - rytilahti/python-songpal: Python library for interfacing with Sony's Songpal devices).
There has been some activity but it seems to have died silently (Add Sony Songpal group and ungroup services by maximoei · Pull Request #66911 · home-assistant/core · GitHub).
Could someone please provide a definitive answer whether the Sony Songpal integration is currently able (or unable) to group player devices and play content in sync on a group of devices?