As you've probably already noticed, the closer you (the receiver) are to the source of the sound, the louder the sound is. If you want sound to travel (propagate) a long distance, it needs to start out loud. If we go back to the image of sound as a rock thrown into a pond and look at the pond from the side, you will notice that the wave gets smaller as it moves away from the place where the rock fell (the source of the waves). The wave gets smaller because it spreads out (spreading loss) and because some of the sound energy is absorbed by seawater.
The following sections provide more information on how sound gets weaker as it moves: