Bhashya
एष उ वा उद्गीथः ।उद्गीथो नाम सामावयवो भक्ति-विशेषः नोद्गानम्; सामाधिकारात् ।कथमुद्गीथः प्राणः ? प्रा-णो वा उत्—प्राणेन हि यस्मादिदं सर्वं जगत् उत्तब्धम् ऊर्ध्वंस्तब्धमुत्तम्भितं विधृतमित्यर्थः ; उत्तब्धार्थावद्योतकोऽयमुच्छ-ब्दः प्राणगुणाभिधायकः ; तस्मादुत् प्राणः ; वागेव गीथा,शब्दविशेषत्वादुद्गीथभक्तेः ; गायतेः शब्दार्थत्वात्सा वागेव ;न ह्युद्गीथभक्तेः शब्दव्यतिरेकेण किंचिद्रूपमुत्प्रेक्ष्यते , तस्मा-युक्तमवधारणं वागेव गीथेति ।उच्च प्राणः, गीथा च प्राण-तन्त्रा वाक्, इत्युभयमेकेन शब्देनाभिधीयते, स उद्गीथः ॥
In plain words
This verily is the udgītha. The udgītha is a specific portion of the sāman, not the entire act of singing, because the context is that of the sāman. How is the udgītha identified with prāṇa? Prāṇa is 'ut', because by prāṇa this entire universe is upheld, supported, and sustained. The word 'ut' reveals the qualities of prāṇa, and hence 'ut' is prāṇa. Speech alone is 'gīthā', because the udgītha portion consists of sound, and the verbal root 'gā' means to sound; thus speech alone constitutes it, for no other form of the udgītha can be conceived apart from sound. Therefore, the assertion that 'speech alone is gīthā' is completely justified. 'Ut' is prāṇa, and gīthā is speech rooted in prāṇa; both are thus expressed by a single word, and that is the udgītha.
Close to the Sanskrit
This indeed is the udgītha. Udgītha by name is a particular portion of the sāman, not udgāna, because of the context of the sāman. How is udgītha prāṇa? Prāṇa is 'ut'—for by prāṇa all this moving world is supported upwards, propped up, sustained, the meaning is; this word 'ut' is a revealer of the attributes of prāṇa, therefore 'ut' is prāṇa; and speech alone is gīthā, because of being a specific sound of the udgītha portion; because of the root gā meaning sound, that speech alone is; for no form of the udgītha portion apart from sound is imagined, therefore the determination 'speech alone is gīthā' is proper. 'Ut' is prāṇa, and gīthā is speech dependent on prāṇa, thus both are designated by one word, that is the udgītha.