A person watching and listening to a radio or television program.
Someone watching a movie, play, sporting event, etc.
A group of people assembled to hear a concert, etc.
A large group of people gathered to see a performance, event, etc.
I'm mostly just literally translating dictionary definitions here, but I think this should help you appreciate the differences between the words. A lot of it hinges on the type of medium being enjoyed and the size of the group (single or multiple people).