Hi @apshep
I'm facing a similar problem (although with a now tv box not a chromecast) I have the BT VIP pass but 5.1 surround sound on Now TV does not work any any device apart from the BT youview box (I have a Now TV box and a roku express)
I speak to Now technical support and get very vague answers, but they do say my account is marked as "HD" not "Boost". Which is why i don't get 5.1 surround sound but do get HD. They then say i should speak to BT. I have done that, and I get no help whatsoever, as you can imagine. It really is a vicious circle with no-one taking responsibility.
Did you ever get it resolved?
