A system to synthesize in real-time the sound of footsteps on different materials is presented. The system is based on microphones which allow the user to interact with his own footwear. This solution distinguishes our system from previous efforts that require specific shoes enhanced with sensors. The microphones detect real footsteps sounds from users, from which the ground reaction force (GRF) is estimated. Such GRF is used to control a sound synthesis engine based on physical models. Evaluations of the system in terms of sound validity and fidelity of interaction are described.