Had a bit of a success with HDMI audio and also video acceleration on OrangePi 3. What was needed was to apply some patches from LibreELEC, namely ones from projects/Allwinner/patches/linux/ .
For HDMI audio to work you need 0004-sun4i-i2s-improvements.patch , which contains support for I2S on H6. When done right you should see in dmesg:
asoc-simple-card sound: i2s-hifi <-> 5091000.i2s mapping ok
Also what helped was an IRC chat that Jernej had with mirko some time ago:
For video acceleration you need 0005-cedrus-improvements.patch and compile Cedrus driver. When done right you should see in dmesg:
cedrus 1c0e000.video-codec: Device registered as /dev/video0
For a good measure I applied other patches in that directory as well, some needed modifications to apply to 5.5 kernel (LibreELEC uses 5.4) .
Then FFMPEG needs to be compiled with v4l2_request to actually do the acceleration
To get Kodi GUI acceleration working you need to compile mesa 20.0.0-devel from Mesa GIT. Kodi then needs to be compiled as GBM version. I also applied patches that LibreELEC has for Kodi into my Kodi tree.