Sound is audible vibration conducted through a medium. (Let's not go into the philosophical debate about the sound of a tree falling in a forest with nobody there to hear it; for now, let's agree that anything with the potential to be heard is sound.)
Surprisingly, according to the current thinking of the physicists studying the Big Bang, sound existed before light. In the first 380,000 years of its existence the universe was an opaque plasma of photons, electrons and baryons. There was no light because matter and energy were one, and all the photons were bound up in the plasma; it wasn't until the moment of decoupling when the expanding plasma cloud cooled to 3,000 degrees Kelvin that the photons were released and light came into existence. But there was certainly sound before decoupling because the plasma was a medium and there was plenty of vibration going on as the universe expanded unimaginably quickly, but not completely evenly. Much of this vibration would have been outside the range of human hearing, but if there had been someone there, protected from the superheated plasma and listening, they would certainly have heard some extraordinary sounds: the music of the birth of the universe. We can hear the distant echoes of this titanic sound in cosmic microwave background radiation, which creates the static you hear on a detuned TV or radio.
It's interesting to note that almost all of the world's spiritual paths reflect this actual physical sequence (first sound, then light) in their creation stories, even though the scientists have only just discovered it. The Old Testament has the heavens and the earth formless, empty and dark with the spirit of God hovering (alternative translation: vibrating) over them - and only then does God say: "Let there be light." The New Testament says: "In the beginning was the word." The Hindus say "Nada Brahma", one meaning of which is "the universe is sound." The mystics of Islam, the Sufis, say that all form manifests from sound. Looking further afield, the degree of consistency becomes quite impressive, with sound being placed at the centre of creation by religious traditions from all corners of the globe including Aztec, Eskimo, Persian, Indian, Malayan, Ancient Egyptian, Polynesian, Japanese, Chinese, Balinese, Tibetan and Ancient Greek. This is another vast and fascinating topic to explore.
For those interested in sound/music and spirituality, two great starting places I commend are: Joachim-Ernst Berendt's classic book The World Is Sound: Nada Brahma and The Mysticism Of Sound And Music by the great Sufi master Hazrat Inayat Khan.
Sound is still a primal force, affecting us every day, albeit often without our conscious awareness. We need to pay it a lot more respect.
Author of the book "Sound Business"; Chairman of The Sound Agency; BrandSoundTM strategist; Conference speaker