I remember blowing into these things back in my college days. I remember that it took a big lungful of air all at once and my lungs were quickly depleted. This makes me think that it takes a certain differential pressure and a large volumetric flowrate. As I remember the sound would rise quickly and then trail off. It made be that the blades inside the whistle operate in an impulsive manner. If someone had a way to measure the pressure and flowrate for reservoir, a nice science project could be done. OK, let's forgo that thought. However, it is interesting to point that even a fairly high wind velocity corresponds to a fairly low pressure drop. For example, I remember that an air velocity of around 70 fps corresponds to a differential pressure drop of only 0.1 inch of water, which to my way of thinking does not amount to very much. Of course, 250 fps will be a lot more, but I tend to doubt if it is anywhere near 1 psia. All of this is not very important if we don't know the operating conditions for the whistle. It is interesting to note that in fluid mechanics power can be expressed as differential pressure (or delta p) multiplied by the volumetric flow rate. So, it might be better rather than have many whistles just have one whistle embedded in a large diameter rocket. A good place would be in the nose cone with a funnel leading to the whistle inlet (similar to a ramjet). At the aft end of the nose cone there must be outlets or ports to allow the air to exit and establish the pressure drop across the whistle.