Measuring Velocity / Amplitude help

Does anyone know what is the best way of measuring velocity from the following patch? -see below-
I'm using a Phidget 8/8/8 with a voltage divider sensor connected to a home made pressure mat. The values I get when a person stands on the mat is either 0 -cero- or 999 -see unpack object-. If I were using a pressure sensor, the input values would have been more accurate, I guess.
The idea of this project is to create a series of pressure mats to be placed on each step of a stair so when people go up or down, a sound will be trigger, which in this case the sound are animal sounds.
The reason why I would like to measure the velocity is because a child weights less than an adult so the sound should be lower, for instance.

there is a video on youtube from where I took the idea from: http://www.youtube.com/watch?v=2lXh2n0aPyw

many thanks