That's just never been an available feature unfortunately. When you cast something to Chromecast, it gets the entire audio and video stream, as that's the way it works. The only workaround I've seen is some users have used a HDMI splitter before the Chromecast to route the audio somewhere else.
This is mostly a user to user community, therefore if this is important to you, I’d encourage you to also send this feedback directly to Google. You will not receive a response, unless Google need more details from you, however the team are still reading the feedback.
