So that gentleman's approach appears to embed 4 cylindrical magnets around the edge of a plastic "O-ring". This O-ring was cut in half and then fastened to the axle of the driver wheel. Then he fabricated and mounted a small circuit board with a magnetic sensor (3-wires) which triggers every time a magnet passes by. So that would generate 4 triggers per revolution of the driver wheel...and of course this matches your description of sound-triggering since presumably you want 4 "chuffs" per revolution (of the driver wheel).
The MTH PS electronics expects the tach pulses at a much faster rate. The gear ratio of the motor RPM to the driver wheel RPM varies between engines. It might be, say, 18-to-1. It would be useful to inquire from that forum if he can manually turn the motor shaft and report the ratio. Anyway, if there are 24 stripes on the MTH flywheel, then with a gear-ratio of 18, the tach sensor receives 24 x 18 = 432 triggers per revolution of the driver wheel! That's over 100 times; so it's the PS electronics which processes these triggers and in this case would initiate the chuff sound every 108 triggers (i.e., 432/4).
@yardtrain posted:
...If the flywheel is not 24/24 how do you adjust the software for that? Is there a minimum number of black and white sections?
As GRJ notes, the standard flywheel is 24/24 stripes. But the gear ratios of different engines varies. The so-called soundset which you download into the engine memory contains the magic numbers on a per-engine basis. The gear-ratio is in this soundset and tells the processor how many pulses of the tach correspond to one rev of the driver wheel. This is how the processor knows when to generate a "chuff" sound. There is an adjustment within DCS where you modify a command-control engine to generate a different number of chuff sounds per revolution. This might assist in some situations and can be explored.
Separately, the processor uses the incoming tach pulses to control speed. Previously, I asked how important it is for the engine to run at scale speed corresponding to the commanded speed. The soundset also contains magic numbers that tell the processor the gauge of the engine so it knows how many tach pulses corresponds to a scale mile. I believe the MTH engines all adhere to their so-called OneGauge which is 32:1. As I recall, G gauge trains can vary widely I think down to maybe 20:1 or so. So if you want commanded scale speed in sMPH to be scale speed, then you'll need to calibrate probably by changing the number of stripes. This would be particularly important if you run MU consists since presumably you'd want multiple engines commanded to the same sMPH speed to indeed run at the same speed. Otherwise, close-enough might be good enough if your consists always have only a single prime mover so to speak.
I know there have been efforts to decode the magic numbers in the MTH soundsets for the purposes of modifying them. I don't know where that stands but seems there have been some OGR threads on this topic.
I can drone on - peeling additional layers of the onion depending on whether you are crying yet! 
