We hear when vibrations and sound energy travel to our ears in the form of waves, vibrating the tiny bones behind our ear drums. Sound is one of the most powerful of the five senses, eliciting powerful images in our imagination in situations where our other senses may be impaired.
Even when we rest, our brains are still working and active. The ears are always tuned in to sounds around you even while you sleep, so that the brain is able to ascertain between a loved one coming home late at night, or a stranger trying to burgle your house.
Just like the sound of strange footsteps in the middle of the night signals danger to the sleeping brain, sounds have the ability to trigger emotions and release feel good hormones known as dopamine. Sounds can affect our neurochemistry, it can change our behaviour and moods. This is why the overtones of a singing bowl soundbath can be soothing to the brain and help with depression and anxiety.
For more details on how sound affects us, check out this presentation by Julian Treasure.