"Google Cast for audio embeds the same technology behind Chromecast into speakers, sound bars, and A/V receivers. Just like Chromecast, simply tap the cast button in your favorite music or radio app on Android, iOS, or the web, and select a Google Cast Ready speaker to get the party started," informs Google. There are already many Android and iOS apps that support Google Cast: Pandora, Google Play Music, Rdio, Songza, Tunein, iHeart Radio, Deezer, Rhapsody and more.
Instead of casting music to TVs, you're casting it directly to speakers. The downside is that you need new speakers that work with Google Cast.
"The first Google Cast Ready speakers will first be available in the US this spring from lead brands Sony, LG, and HEOS by Denon with more brands coming later in 2015 with the support of chip makers Broadcom, Marvell, MediaTek and system integrator Libre Wireless. These products will join a growing Google Cast ecosystem, which includes more Android TVs, game consoles and set-top boxes."
In many ways, Google Cast is Google's take on AirPlay with a cross-platform twist. Apple's AirPlay started with audio and then it was enabled for video. There are many AirPlay speakers and you can also use Apple's Airport Express to connect speakers. Maybe Google should release a Chromecast version with audio output, so that you don't have to buy new speakers or use adaptors like this one.