if I understand you right, you want to play video from HDMI and hear sound from an another source. with the Pioneer, I think the feature for mixing a video source with an audio source is in Multichannel Audio setup.
but no, you can't mate an HDMI video source to another audio source directly. HDMI inputs are greyed out in the multichannel audio assignment menu.
& just like your other "pissing you off" complaint, this is no different than other flagship receivers
I just checked the manual for the Denon 4520, their corresponding model to the SC-68:
for their Video Select setting: "Play the picture and sound of the input source."
I quote from the manual -
"It is not possible to select HDMI input signals."
what you possibly can do is change the audio signal selected for that input, check out pg 60 in the manual...
I haven't tried this on an HDMI input, but for any input, the audio signal is selected in the priority of HDMI, Digital, Analog when it's set to Auto. for say your cable box input, which could be HDMI, you can try manually changing the audio input to analog & see what that gets you
I know this works 100% on non-HDMI sources but haven't had the desire to try it for an HDMI source. you can do the forum & yourself a favor & try it & post your results in the SC-68/67 Owners Thread.
I still think you're expecting a modern HT receiver to do all these combinations intuitively and they just don't work that way. Pioneer is not unique. again, if you take the time to read the manuals for the comparable Denon & Onkyo models, you'll find the same "limitations".
receivers from less mainstream CE companies like Anthem, Rotel, Arcam, typically have less convenience features than ones from Pioneer, Denon, Onyko, Yamaha.
Marantz being based on same platform as Denon also bound to same "limitations".
if you're willing to spend $8000-25000 on processors like the Classe, McIntosh, ADA, Anthem, Bryston, Theta you might
find one that has that kind of flexibility, maybe... but not in the receiver market. but even those hi-priced processors are focused more on processing, having the best analog audio sections and not so much connectivity features. you might read the Anthem D2v manual and see what it offers, but it's going to cost you close to $9K. and the new Bryston is also about that much $ and has far fewer connectivity features than you $2000 SC-67.
Denon, Onkyo & Pioneer seem to be the leaders in convenience, i-app, and networked media features in receivers.
off my soapbox
try the audio select feature, button on the remote & front panel, not sure it's in the i-app but it might do what you want. let us know what happens