Verse 2.8.1

अथ सप्तविधस्य वाचि सप्तविध्ं सामोपासीत यत्किंच वाचो हुमिति स हिंकारो यत्प्रेति स प्रस्तावो यदेति स आदिः ॥ २.८.१ ॥

atha saptavidhasya vāci saptavidhṃ sāmopāsīta yatkiṃca vāco humiti sa hiṃkāro yatpreti sa prastāvo yadeti sa ādiḥ || 2.8.1 ||

1. Now begins a discussion on the sevenfold Sāma worship. One can perform this sevenfold Sāma worship in speech. Wherever the syllable huṃ occurs in speech, that is hiṃkāra. Similarly, wherever the syllable pra occurs, that is to be taken for the prastāva. And wherever ā occurs, that is ādi [the beginning].

Word-for-word explanation:

Atha, next; saptavidhasya, [a discussion] on the sevenfold [Sāma worship]; saptavidham sāma upāsīta, one should perform the sevenfold worship of Sāma; vāci, in speech; yāt kiṃca vācaḥ huṃ iti, in whatever speech occurs the syllable ‘huṃ’; saḥ hiṃkāraḥ, that is the syllable ‘hiṃ’; yat pra iti, where the syllable ‘pra’ occurs; saḥ prastāvaḥ, that is the prastāva; yat ā iti, where ‘ā’ occurs; saḥ ādiḥ, that is to be taken for ādi [the beginning].

Commentary:

The fivefold Sāma worship has been discussed. Now

The first meditation is on speech. Sometimes you may come across the sound huṃ in speech. This sound is to be thought of as hiṃkāra, because the letter h is common in both. Then, when you come across the sound pra, that is to be thought of as the prastāva.

The sound ā in speech is to be thought of as ādi. But what is ādi? Ādi is that with which you begin something. It is Om, for everything in the Vedas starts with Om.