Why Doesn't My Apple TV Have Sound?
If you are experiencing static or no sound, there might be a problem with the Dolby Digital Out setting. Go to Settings > Audio & Video on your Apple TV, highlight Dolby Digital Out and push the select button until it's set to Off. The Apple TV is supposed to be able to detect whether this setting is available but it doesn't work for every TV.
If you have your audio running through an A/V receiver, make sure the Dolby Digital Out setting is On. Turn down your TV audio and adjust the volume through the A/V receiver.