I think most everyone here has watched the beginning sequence to the TV show "Kung Fu" one too many times...you know, the part where Grasshopper's teacher tells him to walk across the rice paper without noise or leaving a trace. Ukemi is not about making (or not making) noise, it's about taking care of your self. I think the real question in Jasper's initial post was "Gosh, why do I slap the mat?" Let me answer it this way:
I weigh 180 pounds, a large portion of that weight being bone mass (breakable). If you drop a cadaver of equal weight from a height of about 10 to 30 inches, would it make noise? The cadaver is beyond caring whether its bones and skull remain intact, so it does not round its back, tuck its head or divert force by slapping the ground. But I think it would still make some noise. If you drop a heavy sack, it will make noise. If you roll a steel-belted radial tire across the ground it will make some noise. Forget the noise crap.
If you have time to think about how hard you are going to slap the mat, you aren't doing your job as uke. "Well kote gaeshi is okay, but maybe not for shihonage." When do we have time to make up these rules? I agree with Hagen. It's not uke's job to make himself, his teacher, or his ukemi look good by taking "ninja rolls" or "silent ukemi". His job is to get up in one piece. And if you must make noise, make it. I usually make a lot of noise from ukemi the next day, you know, walking around and stuff....