Google has released APIs and SDK release notes for supporting Cast for Audio.
https://developers.google.com/cast/docs/release-notes
https://developers.google.com/cast/docs/audio

There's an obvious feature request here which is an official Chromecast to audio jack/Phono converter to allow a Chromecast to be plugged into a stereo. And (maybe) a checkbox in the Chromecast setup to say that this Chromecast is audio only.