Multi room audio is great. A few years ago, if you hadn’t built your house with speaker wire to every room, you were pretty much out of luck. Running wire after the fact is a nightmare, and the old analog wireless speakers were universally terrible.
Now days bluetooth has made wireless audio pretty common, but generally limited to playing from one device to one speaker or set of headphones.
Sonos is a company that has made multi room audio easy to setup and great sounding, but it’s very expensive, and you’re locked into buying from them if you want to expand.
Apple actually has a pretty good solution to this that’s been available for years, but no one seems to know about it.
Every iOS device, the old AirPort routers and Apple TV supports AirPlay, and apple has licensed the protocol to 3rd parties to implement in their hardware.
These work great out of the box, but they're pretty much limited to one speaker playing back at a time. The only application I’ve seen that will play to multiple speakers simultaneously is iTunes.
Enter Rogue Amoeba with an app called AirFoil. This $20 program runs on your mac or PC and manages all the air play devices on your network. It allows your mac to play to any combination of speakers and set their volume independently. It even has an EQ if you want more bass or treble in your sound. I don't have any relationship with Rogue Amoeba (I still have to spell check it every time I write it), I'm just a happy customer.
Even better, there is an iOS app that lets you control airfoil from your iPhone. Now, wherever you are in the house, you can control the music and volume. If you want to turn the music on or off on certain rooms, you can do that too. The protocol keeps everything perfectly in sync. You'd be surprised how distracting a slight delay between different speakers can be.
Aside from just buying AirPlay compatible speakers, any speaker that supports Chrome Cast can be added to your network of speakers. If you have an old iPod dock and an old iPhone, there's an airfoil app that can turn that into a remote speaker too. They have a legacy app that works on phones at least as old as an iPhone 3G. Finally an excuse to dig them out of the drawer they've been collecting dust in.
I recently added a new room by plugging an old console stereo into an AirPort Express I got used for $20. The stereo only had a record player and barely functional radio before, now it sounds great.