Sound spreads in the medium in the form of concentric spheres known as wave-fronts.
Now, as each wave-front expands, it moves away from the source of the sound and expands at a wider space. This means that its energy decreases and the sound becomes quieter.
So, overall, as sound spreads in spheres:
1- The energy per wave decreases
2- The amplitude of the wave decreases
3- The sound becomes quieter as the sphere moves further away from the source
Hope this helps :)
