Sound is a type of energy made by vibrations. When an object vibrates, it causes movement in surrounding air molecules. These molecules bump into the molecules close to them, causing them to vibrate as well. This makes them bump into more nearby air molecules.
If a sound comes in at an angle to the right of the face, the direction of the sound waves means that the sound will not reach both ears at the same time. The time lag is due to the fact that the distance from the source of the sound to the left ear is a little longer than it is to the right ear.
Musical direction is what separates an actual musical performance from, say, an old-fashioned MIDI file from the 1980's and 1990's. The latter may give you correct note values, pitches, and perhaps some dynamic contrast, but it tends to ignore important qualities such as phrasing and expression.
