By default, Zappiti player is able to convert AAC multichannel in LPCM 5.1. If you want to use HDMI main, you must choose the AUTO mode or LPCM 5.1 as this type of format is not supported in RAW. If that doesn't work in Auto mode, choose LPCM 5.1 in HDMI setup (Settings > Sound and notifications).

Warning !  AAC multichannel is not supported with "HDMI Audio Only" output. To get AAC multichannel format with HMDI audio only out, choose  LPCM 5.1 in HDMI configuration (Settings > Sound and notifications).

But if it does not works on all AV receivers and it is not compatible with SPDIF. In such cases, we invite you to re-encode them using the following procedure:

You only need to use 3 software products and all are freeware (one encourages to donate to support it).

1. Mediainfo (to identify your media file audio)
https://mediaarea.net/en/MediaInfo

2.MKVToolNix (to extract the audio track and to rejoin the converted audio to original movie)
https://mkvtoolnix.download

3. MediaHuman Audio Converter (converts audio track to AC3, various Mhz options)
https://www.mediahuman.com/audio-converter/

How to do it :

1. drag and drop movie file in mediainfo, make a note of audio track settings. 

2. Drag and drop movie file into MKVToolNix

3. Uncheck all files apart from the audio track

4.click start multiplexing and the file should be saved as .mka file (note remember where you are saving to , I found making a folder called "conversion" and draging file into there to begin with made it easy as software defaulted to native file location. 

5. Click on file and edit end extension and change from "MKA" to "AAC" you will get a warning but ignore it it will work fine, then hit enter(I am using Mac OSX, but sure extensions can be changed in windows file manager). 

6.Open media human audio converter drag aac file into select AC3 with Mhz, 5.1 that you wrote down from media info.

7. Click convert (make sure you set saved location as "conversion" folder so not to lose file. When complete drag original file into MKVNix and deselect original audio
file (you can also uncheck audio tracks you dont want i.e. commentary etc to make file smaller)

Drag and drop new AC3 file. and then click multiplex and this will create a new movie file with converted audio, perfect match, no issues.

Process takes as little as 10 minutes up to 20 minutes for large complext audio files but nothing like 26 hours and all in sync, perfect quality sound.