The speakers create sound by transferring mechanical energy to the air by pulsating a membrane at specific frequencies. This pulsating mechanical energy at specific frequencies are "sound waves" and are detected and interpreted by your ears as "sound"...which, if organized into a particular pattern, is interpreted by your brain as "music". At higher volumes, the speakers are trying to generate sound waves of larger amplitude. This requires the speakers to pulsate their membrane more (i.e., move back and forth a larger distance). Because the membrane is a physical material of finite dimension, it can only be stretched so far. If the desired volume requires the membrane to stretch beyond its capabilities, then it will no longer be able to generate the desired frequencies...speakers generating a frequency other than that desired is called "distortion".