Okay, so I figured out the issue... And I'll explain, just in case anyone else finds this thread.
So I was trying to get Atmos, DTS-HD and TrueHD from my PC (Quadro P4000) to my receiver (which isn't 4K HDR but 4K 60p).
My 1st configuration was HDMI👉🏻TV👉🏻HDMI ARC👉🏻Receiver which works for Dolby Digital and the like... But not anything lossless.
Reason being, with the Quadro connected to the TV, the ONLY audio options available are 2 channel 48Khz Dolby Digital and Digital+. This is because the TV is only a stereo device!
For whatever reason, nVidia doesn't display all the audio format (thru HDMI) options... Only the options on the CONNECTED device.
When connected directly to the receiver, all audio formats that the Quadro supports are shown, DD, DD+, TrueHD, DTS, DTS-HD, and all the sampling rates!
Solutions...? Quadro👉🏻TV directly for HDR video (TV volume off)
A second HDMI👉🏻AV receiver.
The real trick is, you need to have both DISPLAYS enabled on the PC, configured as 'extend displays'
This is the only way I've been able to keep HDR thru windows and allow the audio output to the receiver to work.
Now I just configure MC using the audio from the receiver, enable bitstreaming and Bob's your uncle!