The new Chromecast Audio is neat. But the box only comes with a 1/8th inch aux cable (AKA a headphone cable), so if you want to use that handy little streaming gadget with anything that doesn't have a standard headphone jack, you'll have to dive into your rat's nest of ancient cables and adapters. Or you could head for Radio Shack, realize that Radio Shack closed two years ago and you never noticed, then head to Best Buy, ask the guy in the blue shirt where the audio cables are, find him again and ask him where the audio cable that you actually want is, then leave the store in defeat when he admits he's out of stock, then finally go home and wait a week for a $5 cable to come in the mail from Amazon.
Or you could just head to the Google Store. Starting today the company is offering official audio adapter cables for the Chromecast Audio, which come in both RCA (the little red and white round plugs) and optical (the weird squarish plug) varieties. They are yellow. Extremely yellow, in fact, with both the cable and the coverings blaring out in an almost neon hue. They're also pretty expensive: $15 for either the 1/8" to RCA or 1/8" to optical adapter, neither of which is very long (though they don't need to be).
The cables are live on the US version of the Google Store, and will probably pop up elsewhere soon if they're not available already. But seriously, those are really expensive cables. Get them somewhere else so they'll be cheaper. And less yellow.