Let’s say that two objects collide and reproduce a sound with a given frequency or frequencies, what properties of these two solids determine the frequency or frequencies produced ?
There are probably more sources of sound than I will discuss here, but as a starting point...
Consider a tuning fork hitting the edge of a table. The frequency of the sound produced by the tuning fork is a function of its stiffness and mass. The tuning fork will vibrate at its fundamental frequency, but there will also be a small amount of sound at its overtones.
Sound could also be created by the displacement of air caused by the "shape" of the collision. For example, experiment with clapping your hands in different ways. You can get different frequencies depending on how much you "cup" your palms.