When I move from the hallway into my living room, it sounds so open and spacious. Why is this?

In most hallways, you are hearing sounds bounce very quickly against close-facing walls. When you walk into a living room or any larger space, the sounds suddenly fall away as they take longer to bounce from wall to wall. The area around you will now feel more spacious or open to you. This is a basic but useful 'clue' to help orient yourself to your environment.