The macOS High Sierra 10.13.6 update adds AirPlay 2 multi-room audio support for iTunes and improves the stability and security of your Mac. This update is recommended for all users.
AirPlay 2 for iTunes: - Control your home audio system and AirPlay 2-enabled speakers throughout your house - Play music at the same time on multiple AirPlay 2-enabled speakers in your house, all in sync
Other improvements and fixes: - Fixes an issue that may prevent Photos from recognizing AVCHD media from some cameras - Fixes an issue that may prevent Mail users from moving a message from Gmail to another account
I was wondering when iTunes was going to get AirPlay 2 support.