Chanting or repetition of sacred sounds is universal in all religions. In Sanskrit, these sounds are called मन्त्र mantras, and their repetition is called जप japa.
"When phonetic combinations are made with wisdom, mantras are produced. Therefore, a mantra is a wise combination of letters whose sounds determine spiritual, psychic, and also physical effects." —Samael Aun Weor, The Divine Science
Mantras and prayers are commands or requests for aid: what matters is that the intended recipient hears and understands the meaning, so it can be acted upon.
Many words or sounds in our modern languages have opposite or contradictory meanings. So if you say a word in English, that same sound has the opposite meaning in other languages: which one should apply? The meaning in your mind, or the meaning in the mind of the listener? Nature does not recognize either of them: the actual vibration of the sound creates an effect, no matter what "meanings" we have in our minds. Mantras are those root sounds: they create precise effects, and are mostly unrelated to modern languages.
For instance, if you learn a mantra but it is from black magic, you may believe it is good and true, but it is not: it is black. So, your beliefs and ideas about it, your interpretation, does not matter. The pronounced mantra creates the effect that the black lodge wants.
Similarly, when you chant a mantra or prayer that is pure and good, you may not understand it or even have a clue about its meaning, but it will have its effect anyway. However, if you do know the meaning, and you invest your consciousness into each vowel, then power is magnified and extended.
The effects of mantras are produced by the action of consciousness, not by the vibration of the physical matter in the throat. Therefore, someone can repeat a mantra a million times, but if they do it unconsciously, without paying attention to what they are doing, they will get little or no result. On the other hand, someone who knows how to be conscious of using the mantra can use it one time — even silently — and create enormous change.
Hand in hand with this is chastity: a person who conserves and transmutes their sexual energy will have immense power (Ojas) in their words (including mantras), while someone who fornicates will not.
Should Mantras be Spoken Aloud or Silent?
"...the verb is of triple pronunciation and that it endows three norms: verbal, mental, and conscious. One can articulate with the creative larynx, one can vocalize with his thought, and one can vocalize with the superlative consciousness of the Being." –Samael Aun Weor, Esoteric Medicine and Practical Magic
"There are three ways that one learns to use a mantra, to repeat prayers or sounds. They are quite simple: aloud, quietly, or silently.
“The fruits of whispered japa are a thousand times more powerful than the verbal japa, and the fruits of the silent, mental japa are hundreds of thousands of times more powerful than the verbal japa. Mental japa can even be kept up while at work.” –Swami Sivananda
"This mantra is pronounced softly, or with the mind and the heart. It can also be used as a “silent word,” because there are two types of words: the articulated word and the silent word. The silent word is powerful..." –Samael Aun Weor, The Need to Change Our Way of Thinking and Feeling
Consider the effect you are seeking: if the mantra you are using is intended to affect your astral body, then the physical vibration is not as important as the astral vibration, so you should focus on the mantra internally rather than physically. If the mantra is intended to affect the mind, the consciousness, or a chakra, then it is logical that one should be seeking to place the power of the mantra in the appropriate dimension. If you are using a mantra hoping to leave your physical body behind, then one should not be focused on the physical sounds, sensations, or body! This is logical. Thus, those who use mantras aloud while focusing only on the physical world may not reach the goal they seek.
The Sounds of Mantras
Generally speaking, the sounds in mantras are pronounced using the ancient roots (Latin, Sanskrit, etc):
I: as the ee in "tree"
E: as the eh in "they"
O: as the oh in "holy"
U: as the u in "true"
A: as the ah in "father"
M: extended as if humming, "mmmmm"
S: extended like a hiss, "sssss"
CH: if the word is Latin, pronounced as k. If the word is Hebrew, pronounced as a scrape in the back of the throat, as in "Bach"
G: in most mantras, G is pronounced as in “give”
Examples of Mantras Spoken by Samael Aun Weor
The vowel I • Clairvoyance, middlebrow, note TI
The vowel E • Occult ear, larynx, note SOL
The vowel O • Intuition, heart, note FA
The vowel U • Telepathy, solar plexus, note MI
The vowel A • Memory of past lives, lungs, note LA