Dolby has audio and video format specifications. Please verify for messages about Dolby Vision HDR video versus Dolby Digital 5.1 audio. Resolution doesn’t have much to do with audio, they’re largely independent if you don’t hit HDMI bandwidth constraints.
If you have set your Apple TV to SDR and activated Match Dynamic Range, then the video signal will switch between SDR and HDR based on the content, for the intended viewing quality. That switch may take a moment for your TV to process. Higher end TVs will adapt faster.
What is your audio setup? If you have two speakers (left and right), then stereo is optimal.