While I love proper external monitor support AirPlay is still broken. When you’re connected to an external monitor that has any type of “audio” then AirPlay can’t be used. My monitor has a 3.5mm connection on the back so I can’t use my multi-speaker Sonos system with my iPad anymore.
I filed feedback on this when it got busted in beta. See
FB11852693 which remains uncommented on by Apple.