Hi @Se7enair thanks for the feedback!
I am using MusicAssistant myself and it does work today with the ha-raumkernel addon already.
Just add the addon in HomeAssistant. In MusicAssistant, you can add “HomeAssistant Media Players” as an audio device. It will show the Raumfeld devices there.
I have added a bit more context on your Github Issue here: Player Support for Teufel/Raumfeld · music-assistant · Discussion #5328 · GitHub