The Upanishads, Part 1 (SBE01), by Max Müller, , at sacred-texts.com
1. The udgîtha is the pranava 1, the pranava is the udgîtha. And as the udgîtha is the sun 2, So is the pranava, for he (the sun) goes sounding Om.
2. 'Him I sang praises to, therefore art thou my only one,' thus said Kaushîtaki to his son. 'Do thou revolve his rays, then thou wilt have many sons.' So much in reference to the Devas.
3. Now with reference to the body. Let a man meditate on the udgîtha as the breath (in the mouth), for he goes sounding Om 3.
4. 'Him I sang praises to, therefore art thou my only son,' thus said Kaushîtaki to his son. 'Do thou therefore sing praises to the breath as manifold, if thou wishest to have many sons.'
5. He who knows that the udgîtha is the pranava, and the pranava the udgîtha, rectifies from the seat of the Hotri priest any mistake committed by the Udgâtri priest in performing the udgîtha, yea, in performing the udgîtha.
12:1 Pranava is the name used chiefly by the followers of the Rig-veda, udgîtha the name used by the followers of the Sâma-veda. Both words are intended for the syllable Om.
12:2 Cf. Kh. Up. I, 3, 1.
12:3 The breath in the mouth, or the chief breath, says Om, i. e. gives permission to the five senses to act, just as the sun, by saying Om, gives permission to all living beings to move about.