It's not Miles, it's always been that way.
I've made a sound set also and found that of the aircraft engines play at different pitches. I assume this is owing to HTC supplying only a single engine sound as default. To account for the different rpm's from plane to plane, they shift the pitch up or down to suit. Any custom sound put in place of the default sound gets shifted by the same degree.
I make the sound how I want it. Then put it in game and listen to it and reference the pitch to a musical note. Then compare that pitch to itself playing outside of the game. I find the difference in semitones, and then shift the pitch up or down accordingly using a sound editor. Note - the pitch-shift should not preserve duration though as the game doesn't.
There seem to be some filter settings from plane to plane also. Probably related to perspective.
I assume you all know this...just info for any interested.