Ha, the cup mute explains the sound.

Yes, the aah-oow will make a wider aperture than you need. It is establishing the corners and is the opposite of clamping the lips. Greg even suggests that big aperture would be best suited for playing pedal tones. One of the exercises for developing corners is to form the aah-oow around the tip of the little finger. You hug the tip of the little finger from the corners. Be sure not to clamp vertically. You can see this demonstrated in Greg’s compilation of the Foundation level (https://mysterytomastery.com/lesson/the-foundation-level/). Go to timestamp 3:15. There is the first aperture exercise, which is followed by the little finger exercise at 3:22. I believe these are in different lessons. It is probably best to follow sequentially through the lessons.