Furthermore, I have not been able to find any downloadable Dolby Atmos encoded files for testing with my Home Theatre system.
I find demo files that I use on The Digital Theater and Demolandia. They have mostly free movie trailers and branded demo files.
When the wanted files are in .mkv or .m2ts (i.e. not in .mp4), then I try to change containers with Subler, or else convert with ffmpeg, while using passthrough for the compatible streams. HandBrake may also be useful for conversions (easier for users new to using ffmpeg).
e.g. dolby-atmos-trailer_amaze_1080.mp4 (0:01:04)
(1080p version to minimize file size)
Normally, passthrough is supported for Dolby Atmos (EAC-3 JOC) bitstreams.
Right, this excludes the uncompressed “True HD” variant of Dolby Atmos (like Blu-ray), but supports the compressed “Dolby Digital Plus with Dolby Atmos” (like used by streaming services).
Dolby Atmos [Implementation and differences] - Wikipedia
Be sure to use HDMI 2.0/2.1 components in the whole chain, including adapters. Commercial 4K media additionally requires HDCP v2.2 or later for each HDMI component. For HDMI cable, use the 18/48 Gbps rated (Premium High Speed/Ultra High Speed). The component with the lowest HDMI specification may limit what is supported.
However, I can find no way of selecting this option on the QuickTime Player.
Unfortunately, my Mac has an Intel chip, not Apple Silicon, so this type of output is not within my reach at this time, to test with. However, this doesn’t matter at all for in-app AirPlay or Home Sharing to an Apple TV 4K box, where Dolby Atmos (EAC-3) can be supported regardless of Mac HDMI-out specification.
