Re: chromecast audio w/youtube
Not sure that would work unless you have a Paid YouTube subscription (and some users report that it worked on YouTube Music and then it didn't and then it did again and then it didn't again, so it might be something that they haven't really pinned down yet).
Also, from other posts/forums, casting a YouTube tab from PC works with Audio, and also this:
So do this:
-Connect your Android device to the same Wi-Fi network as your Chromecast Audio.
-Open the Chromecast app Chromecast cast icon.
-In the top left corner, touch the navigation drawer More menu icon.
-Touch the Cast Screen + Audio button.
-Select your Chromecast Audio.
And remember, YouTube, in the end, is geared more towards video... so it makes sense that adding/keeping Audio support isn't their priority. Not an excuse, just partial justification.