Well worth reading the Chromecast audio FAQs.
https://support.google.com/chromecast/?hl=en-GB#topic=6279362
Especially, https://support.google.com/chromecast/answer/6279416?hl=en-GB&ref_topic=6279411&vid=1-635795409214628433-1838887243
This one amazed me.
I’m trying to cast audio from YouTube but it isn’t working. I thought this was a Google Cast-enabled app?
Chromecast Audio only supports audio apps and this currently does not include YouTube.
WTF?
Youtube might well come down to some licensing issues. Too bad because it would be a great match for the music features they added into Youtube. Also I love listening to amateur covers on Youtube.
The work-around is to do it from a desktop Chrome browser of course but like most work-arounds this limits functionality a good deal.