I think this is one of those areas where what is ideal and what is practical collide and compromise has to be made, especially when there are other children in the house. We live in an apartment (all on one level) and even in her room with the door shut, DD is still going to be subjected to background noise (hoover, washing machine, people chatting, building site next door etc). However, it does take the edge off the sudden loud noises (DS throwing a metal car onto the wooden floor etc) that would wake her up 20 mins into a much -needed i hour nap.
The breathing thing is a theory. Personally I cant see how it can be right if a baby is napping in a room with the TV/radio on. I mean, can you hear your DH breathing if the TV's on? Course not.