Wow, the conformance between RTC and the value from the remote is excellent. So I think I understand now that the 2nd byte, 0x09, adds a level of precision which eliminates the fudge factor I was struggling with by using 0x08 alone.
One thing I don't quite understand though... All of the diesels have the same size wheels, the same gear ratio, same or similar motors, and in some cases, the same flywheel diameter (if that even matters.) So why would the scale factor in the sound file differ at all among diesels? Does MTH test a prototype, and then set the scale factor based on some observed value? This would make the scale factors very close but somewhat random, perhaps to frustrate attempts at reverse engineering?
Of the locos in your list, the GP7 and GP38-2 would be the closest, in terms of what's under the hood, so perhaps those two merit close examination for differences.