Our entire universe consists of nothing more than frenquencies of vibration. All matter, emotion, and thoughts vibrate within varying frequencies. Modern science, especially with the development of String Theory, has discovered this fact. This is easy to understand when you realize that all electrons, quarks, and other sub-atomic particles are always in constant states of vibration.
The ancient sages of India knew this thousands of years ago, and identified this as AUM (OM):
"Om is the primordial throb of the universe. It is the sound form of Atma (consciousness)." Maiti Upanishad
"In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by him; and without him was not any thing made that was made." John 1:1-3, New Testament
These verses are well know by Christians and are from the New Testament of the Bible. Here the term "Word" as used above is derived from the Greek word "logos" (the New Testament was largely written in Greek) and means word or speech, and vibration is the very essence of speech. This mirrors the Hindu concept of the utterance of OM as the creative force in the universe...
Empirical proof that sound (being a form of vibration, or vibrational frquencies) affects the world around it can be found in the study of visible sound and vibration called Cymatics. First mentioned by Galileo Galiliei in 1632, the study of sound's affects on matter was finally codified in 1967 by Hans Jenny in his book Kymatik (translated Cymatics). Sand (or salt or other similar medium) is placed onto a piezoeclectric plate (a type of transducer or speaker connected to a plate, usually made of metal and also referred to as a Chladni Plate, a reference to an earlier researcher in this field), and a particular frequency, harmonic, or music is played through the plate, causing the material to take various unique, deliberate shapes and patterns as a result.
The use of sound as a tool by humans can be traced back to prehistoric times, when the human voice was used not only of course as a means of expression and communication, but also for the incantations of spells, and eventually for chanting. Chanting is found in many cultures and goes back well before recorded history.
The earliest known use of the human voice in this way is found in the Vedas, the oldest surviving religious texts in the world (the sacred texts of Hinduism). It must be noted that for hundreds of years if not longer before the Vedas were written down they were transmitted orally, teacher to student, for generations. This is also why they are written as mantras (a sacred form of prose), as it is easier to remember large amounts of information this way, but more importantly, it was critical that each syllable was pronounced as accurately as possible. This was because it is believed that the power within a mantra lies in how it is pronounced. In fact, the entire language of Sankrit, the sacred language of India (akin to Latin and Greek in the West) was constructed based on the power of the syllables when pronounced correctly. Each syllable is considered a mantra, including AUM, mentioned earlier.
Yogis and monks, and any other individual reciting a mantra is producing a particular sound vibration that can greatly affect their state of consciousness. Ancient Vedic texts reveal that sound has the power to change the vibrational pattern of consciousness and brings it to another level. This is stating exactly the same thing as sound changing our brainwave states. They knew the power of sound on the mind thousands of years ago. This sound vibration is also used for healing as well. It is the most powerful form of sound therapy used in Ayurveda, the ancient system of medicine from India.
"By sound vibration one becomes liberated." Vedanta-sutra 4.22