are the floors chipboard?
I think so. It looked like that when I replaced the carpet in the shower room.
are they cracked and broken yet?
No, but I think there's be expansion and some bits creak because they have shifted about a bit, perhaps. It's particularly bad on the top landing and in DS2's bedroom over the living room.
how old is the house?
1997, standard construction.
what sort of underfelt and carpets?
Standard entry level underfelt and wool carpets that have been there since the house was built apart from the shower room.
do you wear shoes indoors?
Yes, about 50% of the time probably, but the sound is the same whether you have shoes on or not. It sounds like the ceiling is about to collapse if you are underneath it when someone is upstairs, and when the DCs come downstairs an aircraft landing would probably be quieter. TV sound travels both ways a lot as well, as does conversation. If DS1 is talking with his friends on x-box in his room, which is above my study, I simply can't work. If DC3 is being cared for in the house I can't work either, I have had to rent a study outside the home at great expense just to get some writing done, it can be so bloody noisy.
are the ceilings smooth plastered?
Yes, roughly done IMO as the developer was a cheapskate, but smooth plaster.