I have tested my google chromecast audio devices on hubitat and they work like a charmed, but all that are available on the device command ( picture below) are not available to use on rule, for example the Play, Pause button to resume or pause the current music, nor the volume up and down.
I find it rare because from the command buttons for the device i can do all this with no problem.
In Rule Machine, for Action, Run Custom Action, Capability: Actuator, select your Chromecast Audio device, you will see all of the buttons for using in an action.
On the pic i made the rule for the play and pause button on the Lutron Pico Audio Remote, as long as there is audio streaming to the speakers, it will work.