How to use Dolby Atmos mode in Galaxy S9?

Dolby Atmos (Surround Effect) supports playing dedicated Dolby Atmos contents, so provides high quality 3D spacious sound experience.

Smartphones featuring Dolby technologies offer a richer surround audio experience for your movies, music, and other entertainment. As for normal contents and movies, provide optimal virtual surround effect through real-time sound analysis.

Navigate to Settings → tap Sounds and vibration → Sound quality and effects → Dolby Atoms → tap the switch to activate it and then select a mode  

Pictorial representation of the above settings are as follows:  

Also, you can set Dolby Atoms on the notification panel.