The definition of the word "mantra" : "mananat trayate iti mantrah"
(that which protects us by being repeated and meditated upon). "Tranam" means to protect. All of us is familiar with the words in the
gita: "paritranaya sadhunam" (to protect the virtuous). "Mananam" means
repeating, turning over something in the mind. There is no need to
vocalise the words of the mantra. Even if it is repeated mentally,
healthy vibrations will be produced in the nadis. If the same --the
Vedic mantra -- is chanted loudly ("Vedaghosa") it will give divine joy
to the listeners even if they do not understand the meaning. Such a
sound has the power to make mankind happy.
Mind, speech and body are dedicated to the Vedas when you mutter a
Vedic mantra mentally and vocalise it outwardly during the performance
of a rite involving the body. Of the Vedic rites of this kind yajna is
the most important. What is a yajna? It is the performance of a
religious duty involving Agni, the sacrificial fire, with the chanting
of the mantras. The word itself is derived from the root "yaj" meaning
"to worship", to evince devotion. The performance of a yajna is meant to
please the Paramatman and the various deities.