Strictly speaking spectators usually come to see something. They come to see an event, a spectacle. The people who go to see a sporting event like a football game, a boxing bout or a cricket match are called spectators. Usually in such events the focus is on what you see rather than what you hear. If you are walking along the road and you see two of your favorite stars shooting for a film, then you would be spectator. But when you go to the cinema hall and watch the film which you saw while it was being made, then you become part of the audience. The word audience is related to the word audio. You are at a specific event to listen to something. An audience is an assembly of hearers. People who are part of the audience listen to or watch a performance. When you go to a music concert, a play or a movie, you are part of an audience.
It is interesting that when you go to the stadium to see a cricket match, you are a spectator, but when you watch the same match on television, you are part of the audience.
As you know, there is an exception to every rule. Readers are usually associated with the word audience – although reading has more to do with seeing rather than hearing. This could be because in the old days not many people knew how to read or write. The illiterates had to be read to – they had to listen to other people reading aloud.