Agree... you're primarily losing ATMOS & DTS:X for those who care. With the media player doing the decoding vs. AVR, you're also losing dialog normalization & dynamic range compression, but there's debate on this.
Here's an example.
Here's a white paper from Dolby, which also talks about proper downmixing, but I'd think today's AVR's are smart enough to do this properly. I haven't tested 7.1 tracks in a 5.1 set-up with PCM output, but I'd expect you'd get rears/sides from rears just fine
(or ideally virtual sides somewhere between rears & fronts?), especially with 5.1 as the channel configuration in Kodi's audio settings.
Anyway, hopefully Christian & droidelec are okay with me posting Core's PCM patch results here
(sorry, not sharing the actual patch; that's for them to make available however/whenever they feel best). Test results are too good to not share 
. If bad, would've kept them in private quarters 
...
This is all with
audio output set to HDMI in Android audio settings (not PCM) , and with 7.1 as the speaker configuration (what I have) in Kodi (the preinstalled version w/WeTek OS v1.0.1 OTA firmware).
Initially, I left passthrough enabled in Kodi Settings > Audio Output:
- DD5.1 channel check ... correctly output as DD5.1
- DD+ 5.1 ... output as PCM 5.1
- DD+ 7.1 ... output as PCM 5.1 (not 7.1, as no audio from rear surrounds)
- TrueHD 5.1 ... perfectly output as PCM 5.1
- TrueHD 7.1 ... perfectly output as PCM 7.1
- DTS5.1 ... correctly output as DTS5.1
- DTS-MA 5.1 channel check ... output as standard DTS 5.1
- DTS-MA 7.1 channel check ... also output as standard DTS 5.1
- PCM 7.1 channel check ... perfectly output as PCM 7.1
Now, when I went back to Kodi audio settings & disabled passthrough:
- TrueHD 5.1 & 7.1 are screwed up, with audio only through left speaker, no center, and other channels heard through left
- DTS-MA 5.1 ... output as PCM 5.1, which is great
- DTS-MA 7.1 ... output as PCM 7.1, also great
So, with this patch, this build/version, to get the optimal & proper HD audio output, passthrough should be enabled, but with only
DD/AC3 enabled &
DTS disabled.
One issue I did notice however is that there's no picture, only audio, with certain videos. One I could consistently reproduce the issue with is the LPCM 7.1 channel check clip that Kodi forum member wesk05 shared with me. I've provided the clip to Christian for testing. What's interesting though is when I then tried another video with LPCM 7.1, my full MKV rip of '3:10 to Yuma' Blu-ray, no issue w/video. In fact, playback was perfect (video & audio), and this is VC-1 23.976 video to boot.
