The principles of pronunciation are those general laws of articulation which determine the character, and fix the boundaries of every language; as in every system of speaking, however irregular, the organs must necessarily fall into some common mode of enunciation or the purpose of Providence in the gift of speech would be absolutely defeated. These laws, like every other object of philosophical inquiry, are only to be traced by an attentive observation and enumeration of particulars.
John Walker
The production of any speech sound (or any sound at all) involves the movement of air. Most speech sounds are produced by pushing lung air through the opening between the vocal cords this opening is called the glottis and is located in the larynx (often referred to by the non-technical term 'voice box') through the tube in the throat called pharynx, out of the oral cavity through the mouth and sometimes also through the nasal cavity and out the nose.
What distinguishes one sound from the other ? If you bang a large round drum you will get one sound; if you bang a small round drum you will get a different sound; if you bang a small oblong drum you will get still another sound. The size and shape of the air being pushed around makes a difference. This is also true in the production of speech sounds. When the shape of this vocal tract is changed, different sound are produced.
