Chromecast Audio — much like its video & photo streaming Chromecast brother — allows for easy streaming of audio from your Android device to nearly any 3.5mm-capable speaker (RCA and optical devices are supported as well via an additional cable). Chromecast Audio features an integrated 3.5mm cable that connects to a speaker, letting you stream audio from your Android, iOS, Windows, OSX or Chromebook device via the Chromecast app. Audio is streamed over Wi-Fi, and Chromecast Audio can stream pretty much any audio issued off your device — not just music. That includes Google Play Music, Pandora, Spotify and a multitude of other apps.
Chromecast Audio is available via the Google Store for $35.