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 is a major source of music. Not being able to cast YouTube to a C-A is a bit of an oversight, isn't it?