Hey there GIMJ,
I apologize for the late response, thank you for being patient! Were you able to fix that audio issue? If not, I'd be happy to help out, as this would be pretty annoying to deal with. Now, does this occur when casting from a specific website/file? Either way, make sure your Chromecast is up-to-date on its firmware. Then, make sure both the Chromecast and the laptop are within 20 feet of the router. Finally, try casting from Google Canary to see if the problem persists.
I look forward to your response, and if you have any other questions or concerns for me, please let me know!