The free update makes use of Apple's new AirPlay 2 audio connectivity standard to allow die-hard Apple fans with the cash to buy two of the Siri-powered smart speakers to play back tunes in a traditional stereo pairing.
The new iOS build also includes AirPlay 2. And you'll be able to ask Siri to play a given song in the bedroom or in the living room. And Apple has developed algorithms to deliver balanced sound. That way, they don't have to take up space on your iPhone, iPad, or other device. You can also play different music in every room, with requests like "Hey Siri, play Crowded House in the kitchen", which should be a nice addition for families.
Messages in iCloud is the big one for iOS users.
Messages in iCloud will use end-to-end encryption, and uses cloud storage instead of local storage for message archives and attachments. There've been more Messages fixes over rogue character sequences causing the app to crash, and for conversations to appear out of order. Finally, there's a fix for distorted CarPlay audio, as well as audio over Bluetooth or USB in certain cars.
iOS 11.4 arrives today, and with it, a bunch of welcome upgrades. If you're an iPhone or iPad user, it should be prompted automatically; alternatively you can trigger it manually by going to Settings General Software Update. This feature allows two HomePod units in the same room to stream music in stereo with individual left and right channels.