Bhashya
अथ खलु य उद्गीथ स प्रणव बह्वृचानाम्, यश्चप्रणव तेषा स एव च्छान्दोग्ये उद्गीथशब्दवाच्य ।असौवा आदित्य उद्गीथ एष प्रणव , प्रणवशब्दवाच्योऽपि सएव बह्वृचानाम्, नान्य ।उद्गीथ आदित्य कथम् ? उद्गीथाख्यमक्षरम् ओमिति एतत् एष हि यस्मात् स्वरन् उच्चारयन्, अनेकार्थत्वाद्धात्नाम्, अथवा स्वरन् गच्छन् एति ।अत असावुद्गीथ सविता ॥
In plain words
Now, whatever the udgītha is, that is the praṇava for the chanters of the Ṛgveda; and whatever the praṇava is, that same is named udgītha in the Chāndogya. That very sun is the udgītha, and this is the praṇava. For the chanters of the Ṛgveda, the praṇava is indeed that alone, and nothing else. How is the sun the udgītha? The syllable named udgītha is oṃ; for as one chants and utters it, because verbal roots have many meanings, or because it moves as it resounds, therefore that sun is the udgītha, the sun.
Close to the Sanskrit
Now indeed, whatever is the udgītha that is the praṇava of the Ṛgveda chanters, and whatever is the praṇava, that same is designated by the word udgītha in the Chāndogya. That sun indeed is the udgītha, this praṇava; and the praṇava is designated also as that of the Ṛgveda chanters, not another. How is the sun the udgītha? The syllable named udgītha is Oṃ, this one, for chanting and pronouncing it, because of the multivalent nature of roots, or else going while sounding. Hence that sun is the udgītha.