At its most basic level, sound is movement. The sounds we hear every day are a result of molecules in the air banging into each other -- they react to the impulse that creates the sound in the first place. The molecules in a gas move around in random ways. That speed of that movement depends somewhat on the temperature of the gas. Cooler gases have less molecular movement, which makes collisions within that gas happen more slowly than they would if the molecules were zipping around quickly.
But air isn't the only medium that can transmit sounds. In fact, sound can travel faster through liquids and solids than it can through gases. The molecules in liquids and gases are packed closer together than they are in gases. The molecules also don't move around as much as they do within a gas -- collisions between molecules happen faster as a result.
At 0 degrees Celsius (32 degrees Fahrenheit), sound will travel through air at 331 meters per second. That's approximately 740 miles per hour. But sound travels at 1,450 meters per second through liquid mercury. Solid glass transmits sound at 5,640 meters per second. Generally speaking, the more tightly molecules are packed within a medium, the faster it tends to transmit sound.
Sound radiates outward from its source. Imagine a still pond. Now go ahead and throw a big rock right in the middle. You'll see waves ripple outward from the point of impact. That's similar to how sound travels -- it moves out in waves in all directions. The further from the sound's source you are, the quieter it will be as the waves lose energy and spread out.
Sound waves vary in frequency and intensity. Higher-frequency sounds have a higher pitch. The volume of a sound depends upon how much it changes air pressure levels -- bigger changes mean louder sounds.
So how is it that we hear these molecular movements? We have our eardrums to thank for that. The eardrum is a thin piece of skin inside your ear. When colliding molecules hit your eardrum, it vibrates. Tiny bones connect to the eardrum and transmit these vibrations move along to the cochlea, a structure in your inner ear that contains fluid. The vibrations exert pressure on the fluid within the cochlea and the organ of Corti, another structure within your inner ear, translates these changes in pressure into electrical impulses that travel along the auditory nerve to your brain. Your brain then interprets these signals as sound.