Can't respond in the original post. I looked at some of the sound files on MTH web site. Doesn't look like there were many RK 5V GS-2 made other than Bantam. And no upgrade files either for your specific engine.
I personnally don't think it is a corrupted sound file. It would be worth asking MTH if there were known issues with your model.
There is a Northern file that is different then yours in the RK version. Model 30-1285-1. I would try that one.
It does sound like the tach reader is only signalling every other stripe which could cause your twice as fast; or the processor (opamp) is only counting every other one. I think syncronized chuff and smoke puff would be off also.
Since this is a 5V board you don;t have many options. Try reload a new sound file, replace Tach tape, replace tach reader. Some early PS-2 5V systems needed a 10K ohm resistor installed on the output of the tach reader return (orange wire); but that was for chuff in neutral.
This is an interesting issues. G
