Following the release of iOS 11.4 and watchOS 4.3.1, Apple today began rolling out tvOS 11.4 to the Apple TV which adds AirPlay 2 compatibility to the device. With AirPlay 2 on the TV, you’ll be able to ask Siri to play music through multiple outputs including your television and any other speaker you have paired to the iPhone Home app. You’ll also be able to control playback through the same familiar AirPlay interface you’re used to.
To install the new software on your Apple TV, head over to Settings > System > Software Updates > Update Software and hit the download button. Note that the installation may take a while so be patient.