The Speed Table Method may or may not appeal to you because it does affect the top end speed of the locomotive. You may not find a suitable combination of speed and notching rate.
The premise is simple and works because ESU has linked the sound files directly to the speed table.
The method is counter-intuitive... you have to make the model run slower to make the engine sound notch up faster.
The LokSound sound file is directly linked to the 255 bits used for speed control.
The transition settings between each successive notching .wav file is assigned a speed step value that corresponds to the speed step settings of the throttle in a table in the sound schedule, the Transition Table.
See the Transition Table Method below for more details about this table.
As you adjust the speed of the model using the throttle speed steps, the sound transitions up and down when the speed crosses the threshold point of each notching step in the Transition Table.
The bits are set via CV 2, 5 and 6. They can be fine tuned via the Speed Table CVs 67-94.
The default setting for minimum speed via CV2 is 0 and maximum speed via CV5 is 255.
Therefore, the range of the speed table is 0 to 255.
When you set CV5 to a lower value, say 50, you effectively re-scale the range of the speed table from 0 to 50.
The 0-255 steps in the sound file remain constant through the range of the table but they are effectively compressed between the min and max speeds.
Now the prime mover will notch up faster relative to the speed of the locomotive.
If you have speed matched your locomotives and limited the top speed using CV5 and 6, you have already applied this technique and can do nothing else though CV editing to alter the Notching Rate.
Additionally, this concept may work with other brands that do not have a specific adjustment since the underlying premise is the same, the implementation may need to be adjusted to the specific model.