Beamer will offer the Apple TV a choice of playing either surround or stereo sound. Which choice the Apple TV makes is based on the setting 'Dolby Digital' (In the Settings app on Apple TV, go to 'Audio & Video')
If you don't hear any sound or only a buzz, the Apple TV has picked the surround sound track. Make sure that your receiver is set to receive a Dolby Digital (AC-3) signal.
Alternatively, if you don't have a surround sound setup, set the 'Dolby Digital' option to 'Off'.
For more information about troubleshooting audio issues with the Apple TV, please see Apple's knowledge base article
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