Spreading sound waves weaken because their fixed energy dilutes over an expanding sphere

Evidence

Pressure variations originating at the end of the small tube propagate outward in concentric spheres, which are the three-dimensional analog of the circular surface waves resulting from dropping a pebble in a pond. It is intuitively clear that the pressure variations will become “diluted” and less intense at greater and greater distances from their source. This makes sense because the pressure variations carry the energy required to create them. As they travel away from the source, no new energy is added, so the waves must weaken as they spread. The total energy taken over the expanding sphere remains constant. If δp(t) is the difference between the air pressure at a point in space and the ambient air pressure at time t, the average over some time interval is proportional to the energy of the sound at that point. — Eric J. Heller, Why You Hear What You Hear- An…, 140