If they've been deaf from infancy, basically the entire hearing center of the brain is non-existent. So they'd be hearing sound, but processing it into meaningful content would not happen, if at all. So basically, its like having a cacophany of sound that you can't filter and process...

As for others, one thing hearing people, particularly monolingual hearing people, don't understand very well is that hearing != understanding. Just because you hear a sound doesn't automatically equate to it having meaning. The default for many people is to just SPEAK LOUDER and slower, which does not help in the vast majority of encounters