Cheap adaptor connects Chromecast to amps and speakers

For less that a tenner you can pick up this handy HDMI to Audio adaptor that lets you plug Chromecast straight into your audio amp or powered speakers. 

I've tried it through the line-in on my Squeezebox devices and straight into my AudioEngine A2 powered speakers and it works like a charm. So now I can make full use of my Google Play Music All Access subscription to stream all that music without needing to have the TV on.