From my understanding of Physics and Mathematics, a few things take place. All light travels at the same speed, but some light streams have different frequencies then each other. Consider how the brain interprets light signals. An object either absorbs or reflects light, and when light reflects or refracts off of an object at our scale of mass, not planetary or galactic size where gravity takes effect, the angle the light reflects at is the same angle it leaves at. Example: A wall standing upwards at 90 degrees and light from a street light hits it at an agle of 30 degrees. At the point of intersection if you take a perpendicular bisector to the wall so that it is horizontal with the ground, the light travels 30 degrees below that bisector in the opposite direction it came in from. Well this is how our eyes react to light, and given two light streams to produce another is the forming of pure light or white light. Where the speed of light is constant, the speed of sound on the other hand is ONLY constant within the medium it is in. Sound travels faster through gas, than it does water or a preceived to be solid object. The more dense an object the more the acceleration of sound decreases. We have to consider quantum mechanics as well. The actual electrons or charges of the particular atoms that are in that medium comes into play, while sound and friction can almost be interchangeable, either way you look at it they are both virbrations where light is not. The same wave models that represent the electron cloud containing each of its orbitals/electron pairs with their electron spins, "I belive is the effect of sound", causes wave interference. This is why you end up having sounds at different velocities and each sound also has their own frequency as well as amplitude. Light does have frequency like sound, but has no amplitude. I tried to explain this in the easiest way possible. A simple way to visualize this would be speak a word and listen to how long you can here it. It dissapates. Then take a flashlight into a dark room, and turn it on against the wall. (Provided the batteries don't run out), The light will always show against the wall. Your voice litterally travels all the way through the wall and out the building, where as light is reflecting back a percentage of its intensity too you. Some sound does bounce off of objects but sound does move through all mediums. I theorictally believe that sound itselt moves into eternity or infinity, only that it gets smaller and smaller and smaller forever, anotherwords its velcotity and accleration is decreasing as its limit is approaching 0. I believe this because no matter where you look in the Universe 0 degrees Kelvin is only Theoretical. All things are moving hence friction or vibration and there is sound. This is why it is important to watch what you say. The cosmos are recording your voice.