Answer:
You have probably heard that sound is really vibrations. The sound of a person shouting will propagate as a longitudinal pressure wave in the air in all directions. (That's a fancy way of saying vibrations in the air, but longitudinal waves are important to understand if this is for a class.) Think of the sound as a sphere of air molecules pushing against the sphere of air molecules surrounding them. As the sound travels outward, the radius of this sphere is getting larger and larger (increasing at the speed of sound). As that radius gets bigger, the amount of momentum that pushed the air molecules at the origin of the noise (in the mouth of the shouter) now has to push against a much larger sphere of air molecules. The surface area of a sphere of radius 10 is 4 times bigger than a sphere of radius 5 (SA=4*pi*r^2), so there is about 1/4 as much momentum being transferred to each molecule at radius 10 compared to radius 5. This gives rise to the "Inverse Square Law", or the idea that the intensity of sound decreases by a factor of the radius squared.
Put simply, the intensity of the vibrations in the air decreases faster and faster as the sound gets farther away from the source until the momentum from the initial sound source is so dissipated that listeners far enough away cannot even detect the sound at all.
