I do understand the logical inconsistency in my question but I thought I would ask if anyone has succeeded with a solution for this.
I want to cast a dashboard to my Google Nest hub. This I have done successfully. On this dashboard I have a button to start a radio channel to cast to, yes, the same Google Nest hub. Obviously pressing the button would open the media window, but I would like the sound to be in the background and the dashboard is the only thing you see.