The new Google Chromecast Audio is designed to make it possible for you to construct your very own multi-room audio system using WiFi.
In order to make this happen, you'll also need WiFi dongles in each room in which you intend to play music. You can then use the Chromecast app to group the rooms together. At this point, all you have to do is cast your music to your speakers and see it being reproduced in all your rooms.
The Google Chromecast Audio was already a pretty great and revolutionary product when it first came out just a couple of months ago. However, the addition of new functionality proves that this line of products is not going to sit content on its laurels and is going to constantly be reinvented to give customers new ways to enjoy audio at home.