Google has started selling a couple of cables for the audio-only Chromecast Audio.
The first cable is a standard RCA cable that will plug into almost any pair of speaker system. The cable is about a foot long and comes in a violent shade of lemon yellow.
The second is an optical (or TOSLINK) cable, which is a digital interface and should provide a cleaner output for connecting to an AV receiver or amplifier. This one is also a foot long and comes in the same shade of lemon yellow.
The cables are pricey and cost $15 for each. You can probably get cheaper and longer cables on Amazon that are probably just as good, if not better.