It seems we finally figured out all the missing bits and pieces to get HD audio passtrough working on the RPi4. Huge thanks go to popcornmix for taking care of the broadcom vc4 stuff!
If you are in the mood for highly experimental, bleeding edge pre-alpha testing here's a build you can try:
Please use a spare SD card and do a fresh install with the image. This build is based on LibreELEC master with Kodi 19, config.txt has changed and there's no easy way to go back to the stable LE 9.2 series except restoring a full backup.
Also note that there are a couple of known limitations and issues with current RPi4 LE master builds:
- No support for 4k output, this isn't implemented yet in the new driver. Neither is HDR output. Playing 4k HEVC files with HD output should work fine, though.
- H264 hardware decoding is currently disabled as it can crash kodi when seeking or when playback ends.
- Kodi 19 switched to Python 3 so be prepared that your favourite addons may not be available or work correctly.
To get HD audio passtrough you have to enable it in Settings->System->Audio. Change the settings level to advanced or expert and then enable passthrough in general and the separate AC3, DTS, TrueHD and DTS-HD options (depending on what your AVR/TV supports).
Also make sure you've setup your whitelist correctly and set "Adjust display refresh rate" to "on start/stop" or always in player->video settings to get smooth playback.
As RPi4 HD audio is in a very early stage we'd be glad about any feedback - both working and non-working reports.