Siksa deals with "uccarna", "svara", "matra", "bala", "sama" and "santana". The sound of each mantra is determined with the utmost accuracy. How different sounds have their source in different parts of the body and how they are vocalised, all such details which are of scientific and practical importance are dealt with in this Anga. If it says, "Join your lips in this way and such and such a sound will be produced as you speak", you may verify it for yourself in practice and find it to be true.
Here I am reminded of an interesting fact. The lips come into use in "pa", "ma", "va". They are not used in "ka", "nga", "ca", "na", "ta", "na", "ta", and "na". A poet has composed a Ramayana which can be read without using your lips. It is called "Nirosthya- Ramayana". "Ostha" means "lip". "Austraka", the word for camel, is derived from it and the Tamil word "ottagai" has the same origin. "Nir-osthya" means without lips. Nirosthya-Ramayana was perhaps composed by its author to demonstrate his linguistic ingenuity. But another reason occurs to me. The poet must have been very much concerned about ritual purity and felt that the story of Sri Ramancandra must be read without bringing the lips together.
There is a beautiful verse in Paniniya Siksa(its author, as the name itself suggests, is Panini) which tells us how careful we must be in pronouncing Vedic syllables.
Vyaghri yatha haret putran
Damstrabhyam na ca pidayet
Tadvad varnan prayojayet
"The Vedic syllables must be pronounced with clarity. The character of their sound should not be distorted a bit. But no force must be used in vocalising the syllables. There should be no damage done - no erosion of the sound - and no violence should be suggested in the pronunciation. How does a tigress carry its cubs? Tigresses and cats carry their young ones by holding them firmly with their teeth, yet in doing so they do not cause any hurt to the little ones. The Vedic hymns must be chanted in the same way, the syllables enunciated gently and yet distinctly. Panini, the author of the above stanza, has written the most important work on grammar, a subject which comes next(after Siksa) among the Vedangas. Apart from him many others written on Siksa. There are thirty works in this category. Panini's and Yajnavalkya's are particularly important.
Each Veda has attached to it a "Pratisakhya" which examines Vedic sounds. There are also ancient commentaries on them and these too are included in Siksa.
For a general background, please see here