Sound in air is a longitudinal wave, a series of compressions and rarefactions. These are regions where the air pressure is slightly higher or lower than normal. When this pattern reaches a thin solid, such as a microphone diaphragm or your eardrum, the changing pressure pushes the solid back and forth. The solid is made to vibrate at the same frequency as the sound.
The process also runs in reverse. A vibrating solid, such as a loudspeaker cone or a tuning fork, pushes on the air next to it. This makes compressions and rarefactions that travel away through the air as a sound wave.
These conversions only work over a limited range of frequencies. Every solid has mass and stiffness. At very high frequencies the pressure changes come too fast and the solid cannot keep up because of its mass, so it barely moves. At very low frequencies the changes are too slow and gentle to produce a useful response. The human ear works from 20 Hz to 20 000 Hz, and the structures in the ear are suited to this range.