Siksa comes first among the six limbs of the Vedas, the nose of the Vedapurusa. The function of the nose here is not be taken only as that of perceiving smells. It has also the function of breathing; in fact it is one of the organs of breathing. Siksa serves as the life-breath of the Vedic mantras.
Where is the life of a Vedic mantra centred? Each syllable of a hymn is to be enunciated strictly according to its measure. Clarity of pronunciation is what is intended. Apart from this, each syllable is raised, lowered or pronounced evenly -- udatta, anudatta, savarita. If attention is paid to these points, there will be tonal purity. A mantra yields the desired fruit if each syllable is vocalised with clarity and tonal accuracy. The phonetic and tonal exactitude of a mantra is even more important that its meaning. In other words, even though the meaning is not understood, if the tonal form takes shape correctly, the mantra will bring the intended benefit. So the life-breath of the Vedas, which are a collection of mantras, is their sound [the "sound form" ].
There is a mantra to cure scorpion sting. Its meaning is not revealed. Its potency is in its sound. Certain sounds have certain powers associated with them. It is sometimes asked: Why should the sraddha mantras be in Sanskrit? May they not be in English or Tamil? Those who raise these questions do not realise that it is the sound that matters here, not the language as such. If the teeth of a sorcerer were knocked off, his witchcraft [magic] would have no effect. Why? Because the man would not be able to recite this spell properly.
Enunciation of the mantras is most important to the Vedas. What do we do about it? Siksa is the science that deals with the character of Vedic syllables it determines their true nature. The science of the sounds of human speech is called phonetics and it is more important to the Vedic language that to any other tongue. The reason is that even if there is a slight change in how you vocalise a syllable the efficacy of the mantra will be affected. [The result sometimes will be contrary to what is intended ].
It is because of the importance of Vedic phonetics that Siksa has been placed first among the six Angas. It is dealt with in the Taittiriya Upanishad. Its "Siksavalli" begins like this: "Let us now explain the Siksa sastra ". The name of the sastra occurs here as well as in many other Vedic texts with a long "i" ("Siksa"). Sankara observes in his commentary: "Dairghyam Chandasam": it means that the usually short "i" occurs as long [in the Vedas]. (Such examples are to be found in Tamil poetry also. ) I told you that the Vedic language is not called Sanskrit but Chandas. "Chandasam", from "chandas", denotes here a Vedic usage.
For a general background, please see here