Please.. let's dispense with the nonsense
With you engine OFF, outside noises are set at normal amplitude.
As soon as you start your engine (regardless of you personal sound settings) outside noises are automatically attenuated by a fixed decible level (HTC could give exact figure I am sure)
The range at which you can pick up the audible sound of another A/C with your engine running is less than 200 yards.
I think your complaint is unfounded simply based on the range. If you are sneeking up on someone, you should be able to fire by the time you are at 200 yards.
BTW, I fly with external sounds at 100% and most internal sounds at 80% with the exception of Engine which is at 65%, and that is only because the volume level on HTC's vox is barely audible. (And yes, I know you can adjust the level for vox, but it is totally lame to have the engine noise keep dropping out every time someone is talking)