According to the FAQs, the new Chromecast-Audio doesn't support casting from Youtube. This seems really strange for all sorts of reasons but especially, because of Youtube MusicKey/Red.
+Julian Bond I've got both new chromecasts, just got the audio today and it doesn't let you cast from youtube, even though you can cast google play music or w.e. it is to the TV chromecast??? Pretty bizarre, I hope they support it soon.
And just my 2 cents here but I reckon youtube will get support on/before the Chromecast Audio finally gets multiroom support. What I can see a lot of people doing is casting a movie (or youtube) to your TV set and the audio to a separate pair of speakers. This is might be why they're not supporting youtube yet as they need the multiroom tech to keep the audio & video in sync (just like multiroom speakers need to be kept in sync). But saying that I don't see why they couldn't for now just let you cast the audio only
Julian Bond - 2015-10-22 06:58:46+0000 - Updated: 2015-10-22 07:10:35+0000
http://youtube-global.blogspot.co.uk/2015/10/red.html
Hidden in there, "Youtube music coming soon"