I had initially set up the audio to Dolby Digital 5.1 and it seemed to work. But Recently bought some movies on iTunes and the audio does not play. I then went back to setting>audio and changed it to best available and now it works.
I have my ATV 4k connected to my Vizio tv and the optical audio cable from TV to my vizio soundbar.
I have my ATV 4k connected to my Vizio tv and the optical audio cable from TV to my vizio soundbar.