The use of sound for healing is ancient. Through the Greeks, especially Pythagoras, we know that there is an intimate connection between music and mathematics. Greek healing temples were built around this relationship, being designed so that the sound would carry in a specific geometric pattern within the room.
What we call a spa experience today was preparation for a healing session for the ancient Greeks. The person seeking healing would bathe, be surrounded by healing essential oils, and spend time in contemplation in preparation for the musical part of the treatment in the inner part of the temple. The lyre, considered the most sacred instrument, was used along with flutes and zithers.
Similar geometric designs were used in churches built during the Middle Ages and Renaissance to enhance the effects of the music.
Therapeutic Indian music has come from three sources: astrology, psychology and mathematics. The science of the ancient Vedas was the science of the elements: ether, space, fire, water and earth (Ayruveda uses this same science). Hindu musicians wrote Ragas which were designed to be played or sung at a certain time of the day or night, or in a certain season. These Ragas could then be prescribed for various ailments. “Music, for an Indian, is food for the soul and thus is healing in a spiritual way as well.” 
The Himalayan singing bowls that I use are bronze resonant metal, generally tuned to a familiar note, such as G, but possessing so much more than the single note. A good bowl will have a balanced combination of several male and female voices, giving the microharmonics that can deeply affect the nervous system.
Hearing these tones can help the brain move out of the active beta state and into alpha, theta or even delta states, thus promoting relaxation and sleep.
The Mysticism of Sound and Music, Revised Ed., Hazrat Inayat Khan. Shambala 1996.