Hindu Dharma has concepts where all sounds in nature are an expression of divinity, including the sounds that have no meaning, such as sound of waves in ocean, that of thunder and chirps of birds. AUM is one such sound which is believed to be associated with the creation. This is the only sound to have existed before any other sounds in the universe. The 3 syllables + 1 silence can refer to the various states of the human mind, awake, sleep, dream & unity state. It also refers to creation, sustenance and destruction of the universe itself.