Verse 1.5.1

अथ खलु य उद्गीथः स प्रणवो यः प्रणवः स उद्गीथ इत्यसौ वा आदित्य उद्गीथ एष प्रणव ओमिति ह्येष स्वरन्नेति ॥ १.५.१ ॥

atha khalu ya udgīthaḥ sa praṇavo yaḥ praṇavaḥ sa udgītha ityasau vā āditya udgītha eṣa praṇava omiti hyeṣa svaranneti || 1.5.1 ||

1. That which is udgītha is also praṇava [Om]. So also, that which is praṇava is udgītha. That sun is udgītha, and it is also praṇava, because it seems to say Om [or, has the word Om in its mind] when it appears.

Word-for-word explanation:

Atha, next; yaḥ khalu udgīthaḥ, that which is udgītha; saḥ praṇavaḥ, is praṇava [Om]; yaḥ praṇavaḥ saḥ udgīthaḥ, that which is praṇava is also udgītha; asau vai ādityaḥ udgīthaḥ, that sun is udgītha; eṣaḥ praṇavaḥ, it is also praṇava; hi, for; eṣaḥ, the sun; om iti svaran eti, appears with the word Om in its mind.

Commentary:

This is how udgītha is to be worshipped: According to many scholars of the Ṛg Veda, udgītha is the same as praṇava (Om). What is praṇava to them is udgītha according to the Chāndogya.

Similarly, according to many scholars, the sun is udgītha and also praṇava. How can the sun be referred to as udgītha? When the sun appears, it looks as if it is saying Om, or it has Om in its thoughts.

According to Śaṅkara, the word svaran may also mean ‘uttering.’