No audio when playing mp4 video from MacBook Pro to Apple TV
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)