When two molecules collide, does it produce a sound? When we are in an empty room with no one around, we don't hear any sound, but there are billions of atoms and molecules that are colliding at the same instant.
So my question is, when two molecules collide, does it produce a sound?
 A: A 20 kHz sound wave assuming the speed of sound is 343 m/s has a wavelength of 17 mm ($f\lambda = v$), much larger than the mean free path of molecules in air (roughly 68nm), the wavelength you might ascribe to such a collision. This means that the frequency of such a collision would be higher than the upper limit of human hearing. Such a collision would release energy < $3k_{B}T \sim 10^{-20}$ J.
In practice, sound waves are a solution to hydrodynamic equations which assume a fluid approximation where we are already not allowed to treat the air as individual molecules, but as a fluid.
But even in a situation where you allow sound to be defined for individual molecules, the frequency (in any contrived way you'd be forced to define it) would be too high to detect, and the energy would be too low to detect, for a human ear. In a situation where you inject a single high energy collision, random collisions would quickly sap the energy. Think about how slowly heat diffuses through air, and how the energy has been spread out across the entire volume of atoms by the time it reaches your ear.
Indeed, we do not hear heat, especially because it manifests as a large number of collisions upon our eardrum which provide (in the fluid approximation) a constant pressure.
A: No, not in the same way that collisions of larger objects do.
The reason for this is very simple: at the scale of individual molecules, we are effectively "in space", as in like what a spaceship flies through. If you want, this is inner space, by analogy with outer space.
Sound requires a medium to travel through. Air provides that medium for large objects. But air is made of molecules. At Earth sea level, those molecules are about 68 nm apart on average, but only maybe 0.3 nm in size. In between molecules is vacuum. Thus, the collision of two air molecules effectively happens in vacuum, and so no sounds are produced, for the exact same reason that if two asteroids smack each other in outer space, no sound will be produced.
Now that said, there is a caveat here: "molecules", technically, do not have a strict upper limit to their size. In theory, a diamond, of visible macroscopic size, could be constituted entirely by a single molecule (though realistic diamonds will likely have several). If one were to bump two such single-molecule diamonds into each other, one would then, of course, hear a sound.
A: A sound wave is a synchronised movement of millions and millions of atoms or molecules. The random collisions of atoms or molecules are not synchronised and do not produce a sound wave.
A sound wave is like a stadium wave in a large sports stadium. You only get a wave if people move in a synchronised way, each person standing up just after their neighbour. If people just stand up and sit down at random then there is no wave.
