Sri Aurobindo translates Chhandogya Upanishad, Chapter One, Section Three, Verses 6-7: “Thereafter verily ye shall worship the syllables of the Udgitha saying Udgitha and Prana is the first syllable, because one riseth up with the main breath and Speech is the second syllable, because they call Speech that which goeth forth and food is the third syllable, because in food all this Universe is established. Heaven is the first syllable, the middle air is the second syllable, earth is the third syllable. The Sun is the first syllable, Air is the second syllable, Fire is the third syllable. The Samaveda is the first syllable, Yajurveda is the second syllable, Rigveda is the third syllable. To him Speech is a cow that yieldeth sweet milk — and what is this milking of Speech? — even that he becometh rich in food and the eater of food who knoweth these and worshippeth the syllables of Udgitha saying lo even this is Udgitha.”
Just as OM can be understood by the individual letters and the symbolic word as a whole, so also Udgitha (OM) is broken down into its component parts to be understood both in its part and as a whole. In order to express both an outer and an esoteric meaning, the Rishis of the Vedic times went to the root significance of words and syllables.
Sri Aurobindo notes: “The words udgitha and udgayati are words in instance. … in Veda the preposition is still living and joins its verb or separates itself as it pleases; therefore it keeps its full meaning always. In vedanta the power of separation is lost, but the separate force remains. Again the roots gi and ga in classical Sanskrit mean to sing and have resigned the sense of going to their kinsman gam; but in Vedic times, the sense of going was still active and common. … udgitha meant ascension as well as casting upward the voice or the soul in song. When the Vedic singer said ud gayami, the physical idea was that, perhaps, of the song rising upward, but he had also the psychical idea of the soul rising up in song to the gods and fulfilling in its meeting with them and entering into them its expressed aspiration.”
He continues: “This then is the meaning of the Upanishad thta OM, the syllable, technically called the Udgitha, is to be meditated on as a symbol of the fourfold Brahman with two objects, the “singing to” of one’s desires and aspirations in the triple manifestation and the spiritual ascension into the Brahman Itself so as to meet and enter into heaven after heaven and even into Its transcendent felicity.”
Sri Aurobindo has elsewhere described the symbolism of the heavens, mid-world and earth as representing the manifestations of mind, life and body. With this key, the various terms defined herein can be understood.
Sri Aurobindo, The Upanishads, Chhandogya Upanishad, pp.349-366