The speed of sound versus speed of light is totally irrelevant to this issue and only confuses this matter. If you calculate how far away the plane is when you need to hear it then you need to correct for the speed of sound - about 10-20% or so. Otherwise, it is not important.

How many seconds do you need to realize what is happening (Gosh, I'm about to be hit by an airplane) and get out of the crash zone??? If the whistle can give you that many seconds then a whistle is good.

Take your exampe - a 100 meter dash plus reaction time. How fast can you run 100 meters if your life depends on it? Say 12 seconds, plus 3 seconds reaction time, that's 15 seconds. If the plane travels at 33 meters per second (118 km/hour, 73 miles/hour) then you need to start reacting when that plane is 495 meters away (541 yards, 1624 feet).

If you detect the plane by whistle you need to correct for the fact that the speed of sound is 330 meter per second and your plane travels at 33 meters per second. You need to add 10% to those distances: You need to hear that whistle when the plane is 544 meters away, not 495 meters.

The airplane, however, is not 100 meters across. More like 10 for a small airplane. Moving as little as 5 meters should in theory be enough to avoid being hit. Which gives you, maybe 5 seconds: 3 to react and 2 seconds to move. You need to realize something is wrong when the plane is 165 meters away. Add for the speed of sound: 182 meters, 600 feet, 200 yards.

Oblivious to the surroundings as most people are with headphones, I don't think all the whistles in the world would make any difference at all.