Hmm, I've never really thought about this before. I believe that the harder I'm tossed, the more noise I make when I land (at least when a breakfall is involved). I don't intentionally "slap" the mat during rolls or breakfalls, as I've found it actually distracts others who are training nearby.
You hear a loud slap, turn to look, and then get smacked in the head with a Shomenuchi.
Have a good day!