Audio Setup

Passthrough on tvOS 11.3 is broken by Apple, none of the applications are able to use it.

AppleTV Settings

Apple TV -> Settings -> Video and Audio

Surround Sound: Best Quality Available

Forcing Surround Sound to "Dolby Digital 5.1" might solve issues with receiver that do not support multi channel PCM or if the setup uses ARC

Reduce Loud Sounds: Off

Audio Mode: Auto

Ariana=Settings -> System -> Audio output

Number of channels: default 2.0 (stereo output), set according to your receiver/TV setup