Okay, I did a real-world test with noise cancelling with a BookBook 6 cut to fit an iPhone 7. This is using the phone to call an iPhone 6 on T-Mobile, which I believe has enhanced audio quality (sounds like it, anyway), and a TV on as background noise:
With the case folded over against the back of the phone, it sounds perfectly fine. Quite clear.
With the case wide open and not against the back of the phone, it sounded similar. If anything, it actually sounded slightly worse.
Result: At least in my test, noise cancellation problems caused by talking on the phone with the case folded back on itself over the rear microphone are somewhere between not significant and nonexistent.
With the case folded over against the back of the phone, it sounds perfectly fine. Quite clear.
With the case wide open and not against the back of the phone, it sounded similar. If anything, it actually sounded slightly worse.
Result: At least in my test, noise cancellation problems caused by talking on the phone with the case folded back on itself over the rear microphone are somewhere between not significant and nonexistent.