I agree - most people just forget that there are so many other people around.
One of the common problems is when you have a tour group speaking another language in close proximity. For some reason people just don't interpret a foreign language as something people to listen to - and they'll walk right into the middle of the crowd and start talking in English really loud.
Then you get the foreigners (Asians, Russians - usually people who speak no English) just looking at them like "wha....?"
I often shoosh my wife when we are just walking along cruise ship hallways because she will shout out to me to slow down from a distance. She just forgets it is like a dormitory where people are right behind thin walls and trying to nap or whatever.
I have only had to ask strangers to quiet down once, and it was a situation where there were friends sharing a group of cabins who all left their doors open and partied between all the rooms. It would start at midnight sometimes, but they were noisy all the time.
NOw - Miami South Beach hotels - you are bound to get noise problems there.
In any case, I ALWAYS travel with ear plugs.