Many objects in nature can be thought of as "mass-spring-systems" because they are composed of objects which have inertial mass as well as a spring like stiffness. It is natural for mass spring systems to enter into sinusoidal oscillation. These oscillations may create vibrations of the surrounding air (i.e. periodic sounds), as described under "sound propagation" below.
This figure is an animated version of Figure 1-1 of "Auditory Neuroscience"
As you can imagine from experience and intuition, the frequency, or speed, with which the mass vibrates will depend on how much it weighs. One curious consequence of that fact whcih you may not have thought about before is that you can tell something about how much something weighs from how it sounds. Which of these two objects do you think would be heavier?
Easy, isn't it?