I want to be able to select all AirPlay outputs for audio, and send all system audio through all of the AirPlay speakers, like how iTunes can send to multiple speakers. I know there are GUI utilities like Airfoil that can send audio to multiple sources, but I haven't found any that can do it via command line. I've also not had any luck getting an aggregate audio device to send sound to my Mac, AppleTV, and Airport Express.
I've done a lot of searching, and tried utilities like "audio device", but as far as I've been able to work out, I can't select multiple outputs using that, or anything else that I've come across. Any help would be much appreciated!