No audio when playing mp4 video from MacBook Pro to Apple TV When viewing some files with quicktime on my retina MacBook Pro (Sierra 10.12.6), I can view it on Apple TV and the sound works fine.

However some files produce the following behaviour:


Select Apple TV / Airplay in Quicktime. Video appears on TV, but no
sound (on TV or laptop). 
I click on the Sound Pref Pane and select
Apple TV as the output. There is a slight pause, then the video switches back
to the laptop, and the sound now comes out of the Apple TV


This is a similar problem to those reported here and here, but it's not exactly the same because the solutions suggested don't work for me, and this is consistently happening but only for a subset of my video files.

NB this exact same behaviour also happens when watching youtube videos in Safari.
Have you tried adjusting the “Audio Mode" and the “Surround Sound" settings in Settings ➔ Audio and Video? It may be worth trying several combinations of these settings, although typically it is the “Surround Sound" setting that causes most problems for users.

Your TV may not support pass through or you may need to map the audio to the optical output.

If it still doesn't work you might try one of these.

HDMI to HDMI + Audio (Example Only - Not a recommendation or suggestion that this is suitable in your circumstances)