I believe Greg does intend for you to use the mmmaaaaahoooh setting to play the singing C exercises.
He just posted this video yesterday which may help:
I used to believe that the Mmmmmaaaaaaooooooohhhhhhhhh wasn’t meant that literally, just as a means of mentally getting the throat / body open and engaging the aperture corners, and that we should simply put the mouthpiece to the lips however it feels naturally and the key was to engage the aperture corners from the sides inward horizontally (rather than top-to-bottom, clamping down like a clam, which cuts off the air eventually) and relax the throat and play open.
But I have been experiencing a new level of understanding in my playing lately and I feel like I am playing more and more open and forward. Today as I was playing, I was experiencing something where it very much felt as if I was releasing air through a ridiculously open aperture and I was obtaining a resonance that sounded loud but which had very little actual air moving through the aperture. It was a High C, D or E. It wasn’t very loud at first, but the sensation was easy like I was releasing the air through the aperture and letting the air resonate with the inside of the aperture like holdling the sides of a balloon neck open; it was very little effort.
I’m not an instructor or qualified to answer you, but that’s my opinion based upon my understanding and experience. But it’s a hard question to answer; the ideal may be for you to Skype with Greg as it probably depends on precisely what your interpretation is / what you are doing / how you are interpreting it.
I know Greg does say he doesn’t play all over the horn that way; however, if you watch him play, it does appear that he remains forward even in the extreme register.
The sensation I have had lately, and things have been going well, is that I’m engaging a ring of muscle outside of the red part of my lips to engage the aperture corner and I’m reducing the size of the aperture by clinching those muscles surrounding my lips like a ring around a drumhead and leaving the red part of my lips relaxed and able to vibrate freely in the air column–the inside portion of my lips that the air column passes through.
