Apple also updated tvOS and watchOS today.
When I tried out the feature, I soon found out that it requires that both your iPhone and each HomePod be updated to the newest version of their software.
For the Mac, go to Preferences within the Messages app and click the checkbox next to Enable Messages in iCloud. The new update brings in all the features Apple had promised at last year's WWDC event, such as Messages in iCloud and AirPlay 2. The feature can be activated in multiple iOS devices, where your account is synced and thus makes it easier to manage message threads. This will also help users free up space on their iPhones. Scroll down to the list of apps with iCloud support and toggle on Messages. The update allows the HomePod and Apple TV (the two Apple devices that now support AirPlay 2), as well as forthcoming third-party AirPlay 2 devices, to play the same audio throughout your home or only in select areas of your home. We're talking about Sonos, of course, and the Sonos One Smart Speaker offers full support for Apple's new AirPlay 2 that was rolled out on Tuesday along with the iOS 11.4 release.
Also, some manufacturers, like Naim and Libratone, have confirmed that you'll be able to simply run a software update on the speaker to benefit from the new AirPlay 2 features, but others will need you to buy a completely new speaker. Apple says this will deliver "room-filling sound that is more spacious than a traditional stereo pair" In this "each HomePod automatically adjusts the audio to sound great wherever it is placed and sound great together", according to the company. At $199, the Sonos One is $150 less expensive than Apple's HomePod.
Under the "Speakers" section, Apple mentioned the support has been "announced" for those products, but not when the respective audio firms will roll out the multi-room audio tech.