When you play the harmonium, the nagasvaram or the flute, the sound is produced by the air discharged in various measures through different outlets. Our throat has a similar system to produce sound. It is not that the throat alone is involved in this process. How do we speak and sing? Speaking or singing is an exercise that has its source below the navel in the "muladhara" or "root-base' of the spinal column. From this point the breath is brought up in various measures as we speak or sing. The human instrument made by the Lord is far superior to the harmonium, the nagasvaram or the flute. These latter can produce only mere sounds and cannot articulate the syllables a, ka, ca, etc. Man alone possesses this faculty. Animals can produce one or two types of sound but do not have the ability to articulate.
We may gauge the importance of articulate speech form the fact that the Lord has bestowed this faculty only on man. Such a wonderful gift of Isvara must not be squandered or abused in idle gossip or useless talk. We must use it to grasp the divine powers and endeavour to create the well-being of mankind thereby. And we must also try to raise our own Self with it. All these lofty purposes can be served with the Vedic mantras that the sages have gathered from space for our benefit.
If you recognise this fact you will realise why there should be a sastra called Siksa specially for the purpose of guiding us in the enunciation of Vedic mantras. This science as developed by our forefathers arouses the wonder of linguistic scientists even today. It teaches us how the syllables are to be produced accurately and describes in the minutest detail how the passage of the breath coming from the pit of the stomach is to be controlled. Further, it tells us on which parts of the body the breath must impinge and how it must be discharged from the mouth.
In a sense, air going into our body in different ways is a manifestation of the yogic science: it is because of the vibrations caused in our nadis as a result of the passage of our breath that our emotions and powers take shape. There is a saying, "What is in the macrocosm is present in the microcosm. " As mentioned before, the vibrations within us produce vibrations outside also and these are the cause of worldly activities. That is why those who have mastered the mantras have the same powers as those who have achieved yogic perfection controlling their breath. The one is mantrayoga, the other is Rajayoga.
Siksa explains how each syllable of a mantra is to be produced by the human voice, what its tone should be like. It lays down the duration or matra for each syllable. In determining the matra the short and long syllables (the "hrsva" and "dirgha") are taken into account. Siksa also describes how words that are joined together (according to the rules of "sandhi" ) are to be enunciated without breaking them. All such matters as help in the correct chanting of the mantras are included in this sastra.
Siksa explains in very fine detail how the sounds of the various syllables are to produced. A sound like "ka" is to be created from between the neck and the throat; another like "na" is nasal. To produce the sound of 'ta" the tongue should come into contact with particular teeth - this is mentioned in this sastra; so too how the tongue should touch the upper palate for a sound like "na". Phonemes like "ma" arise from completely closing the lips together and those like "va" (labia-dental) are produced using both the lips and the teeth. It is all scientific and at the same time part of mantrayoga and sabdayoga.
For a general background, please see here