If sound is not being projected on your Apple TV, it may be due to an auxiliary equipment malfunction, a settings discrepancy, using the wrong audio mode, or using the wrong surround sound setting.
Auxiliary Equipment Malfunction
Ensure that volume is not muted and that the levels are properly configured by checking your television and/or audio system.
Make sure that any fuses/breakers are functioning properly and that cables are fully seated.
Apple TV Setting Discrepancy
Ensure that the television itself is selected by going to Settings > Video and Audio > Audio Output.
Wrong Audio Mode
Change the option from 'Auto' to 16-bit by clicking 'Audio Mode' on the 'Video and Audio' settings page. This may allow audio to play.
Wrong Surround Sound Setting
Try changing the 'Surround Sound' option by cycling through 'Stereo,' 'Dolby Digital 5.1,' and 'Best Quality Available.'