What you described is the Doppler effect. The point he was making is that the actual speed of the model may not be sufficient to give you enough of the effect.

A real P51 with the Merlin wide open passing in-front of you at 400+mph will create that wonderful sound. (God, I love that sound!) A model of a P51 traveling at 100+ may not be fast enough to give you the shift in the pre-recorded sound.

That is all true but I think the real problem would be to get it loud enough. It would be difficult to get the recored sound up to around 90db in a small space, low weight and within practical power limits. Remember, your recorded sound has to compete with the sound that your motor makes.

One possibility would be to use an mechanical amplifier instead of a traditional permanent magnet speaker system. A small transducer through a wave guide or resonator might produce better results. (Similar in approach to an old style phonograph and that Bose "Wave" radio.) This approach would be lightweight, low power but take up some room. (I thought about using this idea to add realistic machine gun sounds to WWI vintage planes.)

This is out of my field of engineering. Any acoustic engineers out there?