The fourteen worlds constitute an immensely vast kingdom. It has an emperor and all living beings are his subjects. This kingdom as well as its ruler is eternal and it has its own laws. If the kingdom and the king-emperor are eternal, the law also must be so. This law is constituted by the Vedas. Though the kingdom, the cosmos, is called "anadi", it is dissolved and created again and again. The only eternal entities are the Paramatman and his law, the Vedas.
The world comes into being, grows and is dissolved in the deluge. Thus it alternates between being and non-being. The emperor and the law remain eternal. At the time of every creation the emperor, the Paramatman, also creates authorities or "officials" and invests them with the yogic power necessary for them to function. In the yoga sastra is taught the truth that one's ears are not to be differentiated from outward space. When we meditate on this truth we acquire a celestial ear. It is with this ear and with the grace of the Paramatman that the authorities appointed by him obtain the sound waves that are always present in outward space. They were the first to know the Vedas and they are the maharishis (the great seers or sages) of the mantras.
Vedic chanting is a mantrayoga. The vibration in each nadi creates certain feelings or urges in the consciousness. Sensual desire is aroused by some, sloth by some and sorrow by some others. To reverse this, when there is sensual desire there is a vibration in some nadis, and when there is anger there is vibration in some other nadis, and so on for each type of feeling or emotion or urge. We know this from actual experience. When we are at ease there is a special glow on our face and this glow is caused by some nadis being cool and unagitated. There is a saying "One's inner beauty is reflected outwardly on one's face". Our emotions cause their own reactions in our nadis. If we succeed in bringing the nadis under control we shall be masters of our urges and feelings. There will then be no need to depend on any external agency for the purpose.
One way of acquiring control over the nadis is the practice of Rajayoga of which pranayama is the most important feature. Mantrayoga is another. When we vocalize a syllable, the vital breath is discharged through the space intervening our throat, tongue, lips, the upper and lower parts of the mouth, etc. It is then that the syllable is voiced or the "aksara dhvani" produced. Vibrations are created in the nadis located in those parts of the body where the vital breath courses through as a consequence of the aksara-dhvani.
What are the Vedic mantras like in this context? Chanting them means only voicing such syllables as would cause beneficent vibrations of the nadis, beneficent vibrations that would produce such mental states as would lead to well being in this world and the hereafter and ultimately to liberation. No other type of vibration is caused by the chanting of the mantras.
What is a mantra? "Mananat trayate": that which protects you by being turned over again and again and again in the mind. By birth the Brahmin is invested with the duty of chanting mantras again and again and producing such vibrations in the nadis as would bring Atmic well being. Through the power of the mantras he must create this well-being not only for himself but also for all creatures.
How are the mantras to be chanted so that we may master them and derive the full benefit from them? But first let us consider the faulty ways of chanting.
Giti sighri sirahkampi tatha likhitapathakah
Anarthajno lpakanthasca sadete pathakadhamah
"Giti" means one who chants a mantra as he likes setting it to tune, as it were, like a raga. The Vedas must be recited only in accordance with the tones appropriate to them. " Sighri" is one who hurries through a hymn. To derive the full benefit from the mantra the right matras must be maintained in the chanting. "Sirahkampi" denotes one who keeps shaking his head as the chants. There must be a certain poise about the man who chants the Vedas. The nadi vibrations must be such as are naturally produced in the course of the intonation. There must be no other vibrations. If the head is shaken as in a music recital the nadi vibrations will be affected. The "likhitapathaka" is one who chants, reading from the written text. As I have said so often the Vedas must be taught and learned without the help of any written text. The "anarthanjna" is one who does not know the meaning (here one who does not know the meaning of what he chants). All those belonging to these six categories are described as "pathakadhamah" belonging to the lowest types among those who chant the Vedas.
For a general background, please see here