Earlier this week Google came out of nowhere to announce that Google Cast will soon work with participating audio devices, such as speakers, sound bars, etc. It will work exactly the same way as casting a video to the Chromecast. We were able to check out the LG Music Flow system, which is one of the first with Google Cast for audio.
The LG Music Flow system is made up of WiFi-enabled speakers and sound bars that can be powered with batteries. They can be used independently, or synced together. LG has their own app for Android and iOS for controlling the speakers. To cast to the speakers just look for the same icon you use for Chromecasting. Choose the speakers from the list of devices. Check it out below.