It has nothing to do directly with speed.
E = MC squared is - the energy contained within any object, and is equal to, its mass(or weight) multiplied by the speed of light, squared.
This formula enables us to determine and compare the energy contained within different objects.
It works on the basis that light is the ultimate irreducible form of energy within the solar system, and travels at 186000 miles per second approx. Sound is pedestrian by comparison, about 750 MPH. Light is fast because it is the most fluid form of energy, and encounters the least resistance in motion.
I repeat, however, that this formula isn't mainly about the speed of light, it is about measuring the amount of light energy contained within a given object.
Atomic energy (splitting the atom) allows this energy to be released. It was development of this formula that gave rise to its eventual misuse, and hence, the development of atomic power, and the release of this energy in a destructive way.