Hi
The other thing to keep in mind with Now is that unless you have a boost pass active on your account you will be limited to one active stream at a time. So if you don’t have boost when using chromecast, you need to put the app into cast mode before you start streaming content, otherwise the mobile app will start streaming first and will count as your one allowed active stream without boost. If you have boost your allow up to 3 active streams at once.
In relation to audio. There is a specific chrome castaudio set within the Now app itself a mobile/tablet. (Found in my account) This must be set to stereo or casting will not work on the Aura as chromecast built-in, which is what the Aura uses does not like 5.1 audio.
Hope this helps