What is sound? How does it travel? To answer this question, we use a mechanical vibrations approach. We model a mass-spring system and then we minimize the system to an infinitesimal level till we reach the wave equation. The wave equation describes how sound propagates. Therefore, we can see that the propagation of sound is basically just a mass-spring system. It's nothing more than a vibration in the medium (air).
Reference: "What is sound? How can it travel? " - Answer on Quora by Mohamed Mohsen.