I fly with the pmdg 737 and 777, and while the sounds are perfect at low altitude, some of the best I have ever experienced in an fs addon, they are innacurate at high altitude, this is a limitation of the esp platform which does not take air density into consideration with sound, the reason jet engines make that whining and buzzing sound is because of the high density low altitude air, at high altitude the sound is not there. It doesnt have anything to do with power output, during climb at the higher flight levels engine N1 is sometimes higher than takeoff N1, but the sound is completely different. The best fix for this would be for the sim to load separate sound files for high altitude, that eliminate the whining and buzzing sound, but keep the "whoosh" if you know what I mean. For now I think the simplest fix would be for a script or fsuipc itself to be able to adjust the simulators engine sound volume level on the fly, in reference to altitude. As if you were slowly dragging the engine sound slider closer to 0 as the aircraft climbed. It doesn't seem like it would be that difficult, but I dont know much about programming. If anyone can implement such a feature, besides Lockheed Martin, I would think it would be yourself, or anyone familiar with writing .lua scripts.