I have a couple of Apple TVs and an Airport Express in the bedroom with a 3.5mm audio out connected to speakers. The home assistant recognizes the Apple TVs but not the Airport Express.
Wondering if this feature is not available currently or something missing on my part for configuration.
I had the same setup and eventually gave up trying to make a nice solution and went with Sonos. I do have somewhere in an old backup config some switches I used to turn on and off the airport speakers as well as playlists if you’re interested. I exposed them to homebridge so I could control with Siri.
Problem is it gets tricky what words Siri reserved for the iPhone audio playback. Maybe one day Apple will get with the game and actually support their own devices but I lost confidence when I heard they are no longer making airport networking products. It’s too bad, missed opportunity in my opinion. 
Here’s the thread with examples: