Eddie asked: “Is there a multi-room speaker system that works with the Chromecast audio?”
Eddie, the Chromecast audio outputs to a standard 3.5mm plug, so technically it can push audio to any system with an AUX in plug.
That doesn’t mean that any muti-room system will work, you’ll find that a lot of systems assume you want to use them via WiFi so they have their own wireless ways of beaming music to them and don’t provide many extra inputs.
That means that you definitely need to check that the unit you’re interested in actually does have the appropriate input.
Now, what you may be missing here is that Chromecast Audio is muti-room capable, and cheap.
You can buy several Chromecast Audio devices and group them, you can create as many groups as you want, and then stream to a whole group instead of a single device.
That means that if you already have cheap speakers with an AUX input, you can attach a Chromecast Audio to them, form a group and build your own muti-room system for $35 per device.
A single speaker can be a part of multiple groups too, so you could stream to “The whole house” group or just the “upstairs” group, which makes it reasonably versatile for the price.