I have not seen JAWS in ages. But, remember that the music or silence is to build tension and release tension. If you are holding your breath wondering when the shark is going to return, this is tension. When the shark does return, this is a release of tension. The build and release both frighten the audience. When the music is playing, tension builds and the audience is frightened, wondering what will happen, who will die, will the shark be stopped. If I remember correctly, the music seems to get faster the farther along in the film we go. Watch the film for answers, using these thoughts. Music playing does not signify the shark. It signifies that we know the shark is there. Silence makes us wonder where the shark is right now.