Apple have released the iOS 11.4 update for iPhones and iPads. The main improvement is the addition of AirPlay 2 which allows HomePod speakers to play multi-room audio (like Sonos) and to have two of them linked as a stereo pair (again, like Sonos). Another welcome addition is Messages in the Cloud which stores your Messages history in the cloud, not on your device.

Source: iOS 11.4 brings stereo pairs and multi-room audio with AirPlay 2 – Apple