Not a huge deal...sound works when I cast from phone but the whole point i got thjs was to avoid having to use my phone. I tried the troubleshooting steps and it doesn't work. Multiple people are having thjs problem so it's clearly an app issue that needs to be fixed.
I would agree with you that it is an app issue, however as it turns out there are many other apps that also have no sound on this new Chromecast device. I believe this to be an issue with the logic that determined the optimal audio format. I had the same issue on my Chromecast, HBO Max and Disney+ didn't have any sound. I went to the Chromecast Settings and selected Advanced Sound, there I changed it from Automatic to Manual and selected the sound format I needed, in my case this was Dolby Digital + as my TV is Dolby Vision compatible. However you may want to select AVC (stereo). Hope this get's your sound back like it did for me! :D
I have the google chromecast with Google TV and have no issues with HBO MAX . Sound is good 👌
try deleting cache and data , uninstalling , restarting, and then install again.