I think as of this July post, you can't send audio when mirroring your Desktop from a Mac: https://productforums.google.com/d/msg/chromecast/jGSayxcIng4/30jGuul

I think as of this July post, you can't send audio when mirroring your Desktop from a Mac: https://productforums.google.com/d/msg/chromecast/jGSayxcIng4/30jGuullCwAJ https://productforums.google.com/d/msg/chromecast/jGSayxcIng... "Grace Y. said: Hello, Thanks for your patience. Currently, audio is not supported when casting your desktop from a Mac computer/laptop. However, casting a tab does support audio. We're always looking for ways to improve, so your feature request has been duly noted. Best, Grace"