Just to clarify: Throttle is the thin horizontal line on your left central HUD gauge that you move up or down with a, z, -, =, [, ], \, and speed is how full the gauge actually is.
I definitely have to second Chief on this. When it comes to how exactly changing engine (wooshing) sounds impacts retail, it comes down to this:
Existing behavior vs:
1. Hitting A or Z instantaneously silences or maxes out the engine woosh sound. This may sound bad/be weird, or it might be exactly what people want. Hitting - or = may or may not introduce "staircased" volume, depending on how smooth the throttle is capable of changing.
2. When hitting \, I no longer can hear when I actually reach top speed, since the woosh maxes out in volume instantly. Similarly, it becomes harder to tell when I've come to a stop. May be of importance in mods that use ships without HUDS but the retail flight model (BP space suit, for example).
3. We can no longer hear acceleration.
I'm all for making this a feature (sounds very useful/smart/necessary for modified flight models--hell, even gliding is really weird with existing sound behavior), but am all against making this default behavior.