hi, ok I understand better. The treadmill thing is more tricky because this means that the torso z plane won't move. But there might be another way.
One more body movement analysis thing that might help is that while a step is done there is a so called transference o weight. This means that that the center of the two hips moves on the x plane roughly above the foot that is pushing WHILE the other foot is on the air.
Other way is to se that the foot on the air is moving on the z plane in a negative value (goes closer to the camera) while the foot that pushes move in a positive value (in a direction far from the camera. In this case, in pseudo code this would mean
Check for the foot Z AND Y positions : look for the direction inversion of each foot and trigger the sound when a foot z values start to invert.
Beware that the 30 fps of the camera will introduce a little delay between the calculation of the inversion and the sound triggering.
I hope I am clear.
All the best