Use EVENT_FRICTION and phent_getvelocity. Then check the appropriate row of the velocity vector, e.g. for a ground plane just look at phent_getvelocity().Z, if it is above a certain threshold play a sound. It might increase or decrease performance depending on your game, if you do a phent_getvelocity call for each physics entity once per frame and store the values in an array. This way if your phent collides with 3 other objects during the frame you only have to query the velocity once.