Audio Format

If ‘Auto’ gives working audio for your setup, then that is the best setting to use. You should only use the DD 5.1 option if ‘Auto’ doesn’t work for your setup. It is like a forced re-encoding for compatibility reasons, for other setups.

With ‘Auto’, for 5.1 capable receivers, the receiver will get PCM Multichannel 5.1.

For 7.1 capable receivers with 5.1 speakers, the receiver can do the downmix of the rear channels, if a 7.1 steam is output.

Apple TV doesn’t ever send out 9.1; 7.1 is max.

Apple TV 4K doesn’t offer sending out Atmos if it doesn’t detect that capability. For when Dolby Digital Plus with Atmos is the only available audio in the stream, the Apple TV 4K can easily downmix that to 7.1/5.1/2.0/1.0 as needed. For Dolby Atmos capable receivers with 5.1 speakers, the receiver can do the downmix.

If it works for you with surround sound in the ‘Auto’ setting (=PCM Multichannel, uncompressed), then use that. The ‘Dolby Digital 5.1’ settings is a fallback option for when the ‘Auto’ settings doesn’t work in certain setups.