[p3tsin]

#define RUNSPEED 130.0
#define STEPFREQ 0.5
new Float:nextstep[33]

public plugin_init() {
    register_forward(FM_PlayerPreThink,"fm_playerthink",1)
}

public fm_playerthink(id) {
    if(!is_user_alive(id)) return FMRES_IGNORED
    new Float:gametime = get_gametime()
    if(nextstep[id] < gametime && (pev(id,pev_flags)&FL_ONGROUND) && get_speed_f(id) > RUNSPEED) {
        nextstep[id] = gametime + STEPFREQ
        //emit footstep sound
    }
    return FMRES_IGNORED
}

stock Float:get_speed_f(id) {
    static Float:vel[3]
    pev(id,pev_velocity, vel)
    return vector_length(vel)
}

im not sure at what speed u need to be running to play the footstep sound, but i guess 130 is pretty close