[jacanuck]

Does anyone know how to get back multichannel PCM through HDMI?
I'm 100% sure I've been watching movie with AAC 7.1 and my AVR was detecting 'multichannel in" with 7.1 input
Something happened after KODI (or MiBox) updated in the last month. KODI showed that progress bar which appears after update.
After that - multichannel PCM is GONE. Only 2.0 PCM remains and it's really breaking all 5.1 experience with AAC encoded audio tracks :/

In system settings under audio (with expert available) - how many channels do you have set (its near the top of the list).

Normally in Kodi - this is where you tell Kodi how many channels to output via PCM for non bitstream content like AAC.  If you have 5.1 or 7.1 listed, and the Mi Box S is showing more channels than your AVR is receiving - its likely a Mi Box S bug (it was for me for some content, which is one of the reasons in addition to not supporting bitstream content in TrueHD or DTS-HD that I ditched the Mi Box S from my home theater).   

When it worked - I had to set up exact the amount of channels that existed (5.1 for AAC5.1 and 7.1 for AAC7.1) AND turn of passthrough manually. Yes, a lot of effort, but at least I was able to get multichannel PCM.

My AVR is fully capable of receiving 7.1 channels.
Can I ask what do you use instead of MiBox?  

 This setting should contain the number of channels you want when using PCM (when you are using KODI to decode the audio).  If you have a 7.1 system that accepts multi channel input (7.1) then you should have 7.1 listed here.

Passthrough is separate, if your AVR can receive bitstream (dolby digital, dolby digital plus, DTS, etc.) - then you should enable passthrough in addition to the above PCM settings, and only enable the bitstreams your AVR can decode.  Whichever bitstream the kodi box reads in your source content will be converted to PCM should your setting for that codec not be set to passthrough.

IE - if you have an older AVR (Dolby + DTS only, not Dolby plus) - when you watch a dolby plus movie, the kodi box will decode the audio and provide it to your AVR via multi ch in PCM.

Ensure your settings are correct and retest.

I went out and purchased an inexpensive geforce 1030 graphics card and am running KODI on a PC in lieu of the mi box S for now.  The vero 4K+ or nvidia shield are the only other devices I would go with at this time (but others will have more experience with some of the android options out there).