Light travels faster than sound because sound waves can only travel as waves of pressure in a medium, whereas electromagnetic waves, of which light is made, move on their own even through vacuum. Light's speed decreases a little when it goes through various mediums, as electromagnetic waves interact with the medium at a subatomic level. Sound's speed depends on the medium through which it travels.
Sound waves can be compared to ripples on the surface of water after a pebble is thrown into it; the water surface becomes compacted in certain areas and radiating waves can be observed. Sound can only travel through mediums where the density of molecules is high enough to create a wave.
Air is the most common medium for humans to experience the movement of a sound wave. The speed of sound in air, as in any medium, does not depend on the qualities of the sound, such as amplitude, but only on the nature of the medium and its characteristics. For air and other gases, these will include temperature, humidity and altitude.
The denser the medium, the faster sound will travel. Sound travels through air at the speed of 1,115 feet per second, through water at 4,757 feet per second and through iron at 16,404 feet per second. Light travels at 186,411 miles per second in vacuum. This is called the speed of light and is abbreviated as "c." In water, the speed of light drops to 139,808 miles per second.