There is a thin, cone-shaped piece of skin called eardrum inside the ear which vibrates. The outer ear called pinna has a number of curves. This structure helps determine the direction of a sound. The brain interprets the motion of eardrum and we hear sound.
An object produces sound when it vibrates in air. When it vibrates, the air particles around the object start moving. This in turn moves the air particles around the moving air particles thereby creating a travelling disturbance in the form of pulses of vibration. Thus the vibrating object sends a wave of pressure fluctuation through the atmosphere. When the fluctuation wave reaches the ear, it vibrates the eardrum back and forth. Our brain interprets this motion as sound. Since air from the atmosphere flows in from the outer ear as well as the mouth, the air pressure on both sides of the eardrum is kept the same. This pressure balance lets your eardrum move freely back and forth.
The sound wave’s amplitude lets us determine how loud the sound is. More the amplitude, higher is the volume. Sound travels in waves of air pressure fluctuation and we hear them differently depending on the frequency and amplitude of these waves.