The new Apple TV 4 does not have an optical out any longer, meaning all audio has to pass via the HDMI interface. When I replaced my Apple TV 3 with the new version, I found that my surround sound receiver did not play back any DTS or Dolby Digital soundtracks in any of my encoded movies, or Apple TV rented / purchased movies. My Yamaha amp kept on showing PCM mode (HDMI is connected between Apple TV and receiver, the receiver then passes this on to the monitor as a final output of video only) . For all my encoded movies it was restricted to 2 channel stereo, the Apple TV based movies were 5.1 channel PCM.
The fix was to go to Settings -> Audio Settings -> Surround Sound mode to Dolby Surround. This may or may not work for you depending on a thousand factors.