Why do .movpkg files lose surround sound when streaming from Mac to Apple TV?

(1) HLS media (.movpkg) isn’t support for Home Sharing, unfortunately. It affects more than just the Apple TV box; iPhone and iPad also lack these files for Home Sharing. So I think it is already excluded on the source Mac. Let Apple know about your wishes for this: Product Feedback - Apple

(2) Media downloads may not have surround sound, unless specified. See the checkboxes in the TV app Settings, in the Playback tab. You may need to re-download the files with stereo audio, if multichannel audio is available for those.

these forums are user helping other users like yourself, it's sort of going

to a physical apple store and asking other customers there advice

you can give them your opinion using their feedback channels

Product Feedback - Apple

or reach out to their official support channels

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Support (apple.com)

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or

 Contact

- Official Apple Support

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or

Contact - How to Contact Us - Apple

Or

Genius

Bar Reservation and Apple Support Options - Apple

if you mean using airplay then I don't believe that ever supported anything but stereo

if you mean by it accessing a shared lib on a computer and or a NAS then try using

the vlc player app in the app store of the appletv, the normal apple video app support rather few video formats compared to the vlc player

AirPlay Screen Mirroring will have stereo audio, but in-app AirPlay controls should support surround sound. You may still need to specify that track, though.

The Audio Controls in e.g. the TV app has a selectable option in the Audio Control overlay, to ‘Prefer AC3 Passthrough’, when available.

Apple Silicon Macs connected to a surround sound receiver would also have an ‘HDMI Passthrough’ option for audio, which could include Dolby Atmos support.