Yes, we can easily determine the air velocity, because there is a reliable indicator of the air's velocity: pitch. Forget about the trumpet for a short moment. Just think about mouthpiece buzzing: I have quite exactly the same range on both mouthpiece with trumpet and mouthpiece without trumpet. What I can play on my trumpet I can buzz on my mouthpiece (alone), too. And vice versa. My body works and feels quite nearly the same way on both playing trumpet and buzzing on my mouthpiece. When mouthpiece buzzing the pitch raises by raising air compression and making a smaller an aperture by raising the lip tension. At a given pitch (= given air velocity) the loudness raises by raising the flow (air flow per second) which is done by raising air compression and by making the aperture a little bid more open to a degree so that the pitch (= air velocity) keeps constant. Of course you don't (consciously) determine the air velocity itself by (consciously) shaping the aperture and controlling (consciously) the air compression. It's just the other way round: by (unconsciously in a way) controlling the pitch what you do is control the air velocity. The trumpet itself has (almost) nothing to do with it. It just resonates when the lips produce something the trumpet likes to resonate to. .