I will not ever call a foul based on what I hear. I only call fouls on what I see. If the slap caused a disadvantage, then it should be called, not just because you hear skin coming together. And I do not agree about sending a message. I only call what I fell is illegal, not what I think is illegal because I "think" I hear something. What if the slap was while the ball was on the hand? That is not a foul according to the rules.
Peace