I cant speak to everything in your post, but the reduction in volume of the buzzsaw effec5 at the highest n1s is actually correct, its called "coning". During descent some engines produce more bass at at lower n1, at cruise all of the high frequency sounds the engines make should dissapear, so it might sound like the engines get louder at lower throttle settings. This definitely happens irl on some jet aircraft, ive never flown in an a300 though.