Only 2 channels show up under HDMI Output

Hi,

You don't say what OS you're on, but I'm on 10.11.6 and all I get is 2 channel/stereo too, whether I connect from MBP to TV or MBP to receiver. I think that's all we get via HDMI. If you output Thunderbolt/mini displayport to HDMI, then you can get 5.1 audio: Send audio and video to HDMI displays through Mini DisplayPort - Apple Support

You don't need an adapter to get HDMI output because it's built in. That doesn't mean it has 5.1.

Yes, there is a difference between the two outputs, but both easily exceed the necessary bandwidth needed for 5.1. I suspect that a different DSP (digital signal processor) would be needed for 5.1 for any port that they wanted to output it through = $. I also suspect that's why the built in analog/audio output we have in our MBPs also does not support 5.1; there's no DSP for it. So the 5.1 was put in one port, I suspect, because of that.

I wonder if the pic you found is after someone used a Thunderbolt to HDMI adapter, if MIDI then changes its user interface to show HDMI. A lot of suppositions, but unless we find other answers, I think that's it.