In acoustics, this is called the "Far field approximation". But for point further away it works quite well. Keep in mind that this only applies for being "far away" from the source of the sound, because the power/area would be infinite at your mouth, since you would be dividing through a surface area of 0. Now everything has 16 times the power / area in that direction and suddenly you're much louder at 2m distance than you were before in even 1m distance. If both, Assertion and Reason are true and the Reason is the correct explaination of the Assertion. Reason : For diffraction to take place, the aperture of opening should be of same order as wavelength of waves. Instead of evenly distributing the sound over the shell, you're focussing it on let's say on 1/16 of the surface. Assertion : No diffraction is produced in sound waves near a very small opening. That's why your voice gets quiter with distance. So there is only 1/4 the power per surface area than in 1m distance from your mouth. At 2m distance, that shell has a radius of 2m and four times the surface area. In 1m distance, that shell has a radius of 1m. If these waves would evenly distribute around you, imagine all that Energy distributing on a "shell" around you. At medium wavelengths (MF), sound is re-radiated at edges of obstacles.
At long wavelengths (LF), sound passes through obstacles. Let's say your mouth emits sound waves that carry a power P. Diffraction of sound waves is the change in direction of sound waves as they pass through an edge or obstacle. Diffracted sound wave has poor sound tone and quality. It basically boils down to conservation of energy.