Is anyone using the Sony Audio Control API to control their Sony Sonpal / MusicCenter multiroom environment?
There is a NodeRed node:node-red-contrib-sony-audio
Anyone using it? Experiences with multiroom?
These devices can be controlled using the songpal integration. The backend library supports already multiple zones, but there is currently no support in homeassistant for it. Upstream issue for zone support for homeassistant is https://github.com/rytilahti/python-songpal/issues/49 which may move forward after https://github.com/home-assistant/architecture/issues/364 is decided and someone wants to implement support for multizone.
That is bad news 
Do you know whether the multiroom support works in the NodeRed integration?
I don’t think so, as only one media_player is created per device (on homeassistant’s end). For zone support there should be a media_player per zone to allow separate controls.