Is sound a classical mechanic phenomenon or is it a quantum effect? I am just curious because sound is longitudinal waves, meaning the energy is passed from one particle to another nearby particle and produces a wave that continues on until it reaches our ear as a sound wave or hits our skin as a pressure wave, so is this considered quantum because of atoms and molecules?
 A: The detailed physics of sound wave transmission through air were mathematically worked out and found to be accurate before the invention of QM. No quantum-mechanical effects need to be taken into account to accomplish this; it is solidly in the domain of classical physics.
A: Well, niels answer is correct of course. Sometimes you can have also sound in QM system, it is called phonons where a mechanical classical vibration is traveling along the atomic lattice of some materials similar to sound inside a medium.
It is also one of this rare cases where you can treat your quantum system classically.
A: Sound is successfully described using continuum mechanics of solids and liquids (that is the theory of elasticity and hydrodynamics). That is, one need not even descend to the level of molecules/atoms, although it is possible and some interesting phenomena appear at this level.
To summarize:
Sound < Continuum mechanics < Molecular theory < Quantum theory

Quantum sound
As another answer points out, one can speak of quantized sound waves, such as phonons in solids. Even more interesting are such essentially quantum phenomena as zero sound and second sound.
