They do have an acute sense of hearing, so something that doesn't seem that loud to us will seem loud to them. But as the others say, they can get used to general household noises. So for example they may not be bothered by people talking and the TV on while out and about. But they would sense something scary - like if the TV was very loud or people were sounding angry. They do sense peoples' behaviour I think, which is why some rescue hamsters are nervous after not being treated kindly, or handled roughly.
I do think they sense your tone of voice, which is why people talk to them in a gentle tone through the cage at the start of taming. They get used to your voice and will sense whether it is kind or not.
I think the hoover can scare them - but it depends on the hamster and what they have got used to. As Newtie is still very young I don't use the hoover when he's in the cage - although I do have a brilliant old fashioned floorsweeper that picks up bits without making a noise! They are becoming popular again.
Like April Pearl I waited until he was in the other room during out of cage time (popped him in his pet carrier for 10 minutes or so) and hoovered the room he is in then.
With our last hamster I used to do it during the day when he was asleep but it would wake him up and make him a bit nervous. He did get used to it, but I tried not to do it too often when he was in the room.
Very loud noises like fireworks, will scare or startle most pets. Our hamster also likes the sound of some music - but I don't turn it up loud