Sound is a physical experience. Theorized in the early 1800's and scientifically proven in the 1973 paper 'Auditory Beats in the Brain', the field of psycho-acoustic medicine looks at how sound affects mood, learning, sleep & even healing.
Our brains' internal language is based on frequency. When we feel wired or hyper alert we're in a high-frequency 'beta' state, and when we feel relaxed or dreamy we're in a lower-frequency 'alpha' state.
Because our brain has a natural tendency to shift its frequency towards the sound it's hearing, sound therapy focusses on listening to lower frequency sounds proven to stimulate the production of serotonin and neurochemicals associated with relaxation, creativity & healing.
'There's no organ system in the body that's not affected by sound, & vibration .' - Dr Mitchell Gaynor
To experience sound therapy for yourself try listening one of the many freely available low frequency playlists, some 'binaural beats' (where the right & left ears listen to two slightly different tones to stimulate a deeper shift in your brain) or experience a sound bath.