The pitch of the horns is set by the trumpet length, not the diaphragm pressure adjustment screw. They are tuned to a tritone interval, a very dissonant pitch between a 4th and 5th on the scale. Not designed to be regal, just obnoxous.
Here is the skinny on adjusting the standard tactical air horn set;
1..Get some earplugs.
2...Get an assistant to push the horn button when you say so.
3...Use either an allen wrench or screwdriver as needed to turn the adjustment screws. The Delco horns have a locknut you can loosen with a pair of needle-nose pliers.
4...Stuff a rag in one trumpet.
5...Honk horn whilst turning adjusting screw for loudest toot. Don't tighten too much, you may split the diaphragm inside. Don't loosen too much either, as the piston can drop down and you will have to disassemble the horn to reset it where it belongs. What you are doing is setting the amount of air that can creep past the diaphragm/piston causing it to vibrate. Lock it down when it is good.
6...Transfer rag to other trumpet and repeat.
7...Apologize to your neighbors for doing this at 0200 hrs.
Dust or corrosion can get in the way of the diaphragm and cause it not to vibrate. Usually a reset of the internals is all they will need. Sometimes the diaphragm is too far gone and in that case you can make a new one from brass shim stock anywhere from .002 to .005.
You can always make them work again without too much effort.
dg
ps...Two sets hooked up to the same valve will make an unholy noise guaranteed to get more attention than a streaker at the super bowl.