Bhashya
तस्य प्रागुक्तस्य मन्त्रराजस्य सामाभिव्यक्ताक्षरस्य संपुटितत्वेन स्थितस्य ह निश्चितं सामाभिव्यक्तमूलमन्त्राक्षरसंपुटीकरणेन स्थितत्वात् साम्ना संबन्धः, 'तस्मात्प्रत्यक्षरमुभयत ओंकारो भवति' इति श्रुतेः ।प्रणवस्य या पूर्वा मात्रेत्यादि सा साम्नश्चतुर्थः पादो भवतीत्यन्तं स्पष्टार्थम् ।अकारोकारमकारार्धमात्रात्मको यथासंख्यं पृथिव्यन्तरिक्षद्युसोमलोकऋग्यजुःसामाथर्वणब्रह्मविष्णुमहेश्वरोंकारवसुरुद्रादित्यमरुद्गायत्रीत्रिष्टुब्जगतीविराड्गार्हपत्यदक्षिणाग्न्याहवनीयसंवर्तकात्मकः प्रणवः तस्मिन्नृसिंहव्यूहे विश्वरूपन्यायेनावस्थितःउपास्य. ।नन्वनेक एव लीलाविग्रहाः कथं न भवन्ति ।नेति ब्रूम. 'यः' 'तस्मै' इत्येकवचनात् एक एवायं लीलाविग्रह इत्यवगम्यते ॥
In plain words
This mantra, already given, has its syllables voiced through the Sāma, and the two stand locked together -- so the connection with the Sāma is beyond doubt, because the root mantra's syllables only come alive through that very interlocking with the Sāma. Scripture confirms it: 'On either side of every syllable, there stands the syllable Om.' And the earlier verse -- 'the Praṇava's first mora... is the fourth quarter of the Sāma' -- needs no further unpacking; it already says what it means. Here is the Praṇava in full: its four sounds, a, u, m, and the trailing half-mora, and each one carries its own whole world with it, in order -- earth, the space between, the sky, and the world of Soma; the Ṛg, the Yajus, the Sāman, the Atharvan; Brahmā, Viṣṇu, Śiva, and Om itself; the Vasus, the Rudras, the Ādityas, the Maruts; the Gāyatrī, the Triṣṭubh, the Jagatī, the Virāj; the household fire, the southern fire, the offering fire, and the fire that gathers everything back in. This whole Praṇava, everything it carries, rests inside that one array called Narasiṃha the way the universal form holds all things at once -- and that is what is meditated upon. Someone might object: surely a being like this shows up in many playful forms -- why insist there is only one? We answer: no. Look at the grammar itself -- 'who', 'to him' -- both singular. That tells you plainly: this playful form is one, and only one.
Close to the Sanskrit
Of that previously-mentioned king of mantras, whose syllables are manifested by the Sāma, standing in an interlocked condition -- indeed there is definitely a connection with the Sāma, because it stands by the interlocking of the syllables of the root mantra as manifested by the Sāma -- since the Śruti says: 'Therefore, on both sides of each syllable, there is the syllable Om.' [The passage] 'of the Praṇava, which is the first mora' and so on, up to '[this] is the fourth quarter of the Sāma' has a clear meaning. The Praṇava, whose nature consists of the sounds a, u, m, and the half-mora, respectively [correlated with] the earth, the mid-region, the sky, the world of Soma; the Ṛg, the Yajus, the Sāman, the Atharvan; Brahmā, Viṣṇu, Maheśvara, the syllable Om; the Vasus, the Rudras, the Ādityas, the Maruts; the Gāyatrī, the Triṣṭubh, the Jagatī, the Virāj; the Gārhapatya, the southern fire, the Āhavanīya, the Saṃvartaka -- this Praṇava, abiding in that array [that is] Narasiṃha by the principle of the universal form, is to be meditated upon. [Objection:] But surely there are indeed many sportive forms -- how could there not be? We say: no. From the singular number of 'who' and 'to him', it is understood that this sportive form is one alone.