I have released source code of a software package that uses Head Related Transfer Functions to implement two dimensional spatialization of sound. The package has a demo program that runs under Windows to exercise the functions. Here is an example of why my work can be fun. I will try to make it as simple as I can. Grown-ups sound different partly because their heads are bigger.
Sound takes longer to travel from the vocal chords to the lips in a grown-up.
Air is made up mostly of oxygen and nitrogen gas. Oxygen and nitrogen are heavier than helium. That is why helium balloons float. Now, when sound travels through air, it is like waves traveling on water and each wave has to push the air back and forth as it moves along. So, if you are breathing helium, sound arrives at your lips four times as fast as it normally would.