I've always been told by my parents that people can still hear and understand even if they appear unconscious or non-responsive, and so I have a personal policy of speaking respectfully toward them just the same as if they were alert and responding and part of the conversation, but it has shocked and amazed me (and saddened me) by what some people say in the presence of a living person who can very possibly hear and understand but may not be able to respond. Be careful not to be cruel to people in that situation!