I restart Apple TV. In the short experience we have had with AppleTV, restarting seems to be the cure for a lot of issues (frozen screen, frozen show/movie, selection option disappearing, remote malfunctioning, etc). After it comes back on, there is still no sound. I pull down the options screen in Netflix and slide right to Audio. Language, Sound and Speaker are all set to English, Full Dynamic Range, and the correct AppleTV. This menu does not seem to help, so I menu out of Netflix and go to Settings on the tvOS.
Under Settings, I go into Audio and Video. In the next menu, I select Surround Sound. It is set to Best Quality Available. I change it to Dolby Digital 5.1 and menu out to the App scree. I go back into Netflix, and I have the same outcome – still no sound. I menu back out, go back into Settings > Audio and Video > Surround Sound. I select the last option, Stereo, and menu back out to go into Netflix. THE SOUND IS ON!
So though Best Quality Available as the choice suggests it will pick the best available in whatever app you may be in, this is not the case. Many of the shows or moves you watch may be safe under that selection, however, if you are not hearing sound in a selection you have made, try changing the audio down to stereo. That selection may not have a better quality option.
Hope this helps someone out there!