AirPlay 2 brings support for multi-room audio streaming with enhanced buffering. Currently, you'll need to use Apple TVs for multi-room audio; however, you'll eventually be able to use speakers that support the feature like Apple's HomePod or the Sonos ONE.
When installing tvOS 11.3, you'll be asked to assign the device to a room in the Home app. You can then Play or Pause the Apple TV from the app but that's about it.
To select multiple devices for audio playback tap the AirPlay button in the Music app or use the audio widget in Control Center. Being the first beta, we expect this to be a bit buggy to start. You can follow iClarified on Twitter, Facebook, Google+, or RSS for updates.