Someone else posted this tip earlier in this thread - might help you:
"By the way, it's easy to cast audio only from YouTube or any other app - just go to the Google Home app and cast your phone's audio to the device or group."
Not that it should matter too much most of the tine, but that does increase (naively triple) local wifi network traffic since you're streaming from source to your phone, from your phone to your router, and from your router back to the CC Audio instead of straigt source to CC Audio.
Makes sense. Others in the thread had said that Chromecast Audio was way better than standard bluetooth or wireless audio solutions because it had wifi capabilities and instead of requiring battery usage from your phone, it simply pulls the data stream itself from the internet.
"By the way, it's easy to cast audio only from YouTube or any other app - just go to the Google Home app and cast your phone's audio to the device or group."