Simple (?) pressure activated sound

A Piezoelectric sensor is enough for the detection of a kick.

http://en.wikipedia.org/wiki/Piezoelectric_sensor

They are cheap, easy to attach to a shoe and you can even use it as a buzzer to produce the sound.

This simple circuit will help you:

http://www.discovercircuits.com/DJ-Circuits/motionalarm2.htm

I'd try something simpler - you just want to detect when the ball presses on top of the foot, which would require nothing more than a large, thin pushbutton.

Not many off the shelf solutions for that, but you can take a small surface mount pushbutton, sandwich it between two pieces of plastic, and then the plastic surface becomes the pushbutton. You can make this plastic disc as large or as small as you want, which will determine how precisely the soccer player must hit the ball to trigger it.

It should be less susceptible to false positives than many other shock detection solutions such as piezo or accelerometers, which might also be triggered with a particularly hard kick that isn't on the surface of the shoe.