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RoZe_TASTY practice practice practice. Which note are you asking about? When it comes to the high range, I built my range by starting on third space C and going up in half steps until I found a note that was iffy. (Have a fingering chart on your stand, it will help.) the main key to playing high on the horn is air support and the next main thing is you want to raise the back of your tongue as if you were saying the letter E. Hope that helps!

I'm pretty sure its a multiphonic. You can play more than one note at once by singing into the instrument at the right pitch

It's a false overtone. If you play 3rd-space C on the F side of the horn, and press the 1st valve down about 1/4 of the way, you'll pop out a high-C overtone. It's one of the easier tricks in this piece to perform - just play a C and slowly press the 1st valve until you hear it.