Finally there was some momentum to crack audio passtrough support on mainline kernel. Thanks to the RPi4 effort, I gained better understanding what needs to be done. I believe that first variant is ready to be tested.
DTS, Dolby, Dolby+, DTS-HD MA, Atmos
What doesn't work:
Nothing that I could find
[...]Please share your experience, especially if it doesn't work.
Thanks for your -- as always -- great work! I installed it on Orange Pi PC and it works with AC-3 movies without problems. My configuration: Samsung 1080p TV and Onkyo TX-507 AVR. One issue: It only works with movies. I have AC-3 audio only files (recorded from digital radio) and they are converted to PCM. I am not sure whether this is a general Kodi issue and I should file a request at mainline or depending on the implementations, as it seemed during search that the passthrough settings are implementation dependent. If you are the one who sets it, it would be fantastic if you could do that, I assume it is just a few lines to enable only audio files for passthrough that are passed through as audio tracks of video files. AVR wise it is no problem to receive audio only passthrough via HDMI, currently I play those with standalone omxplayer on a RPi 1 (IMHO too slow for Kodi), which I then could "retire".