Every vibration has a corresponding sound and everything in the universe has a vibration and thus a sound.
When you chant a mantra, you merge with the sound vibration and become at one with the energy wavelength of the object of your mantra. All the saints who have ever reached enlightenment through the technique of chanting that mantra connect with you as you connect with the vibration of the mantra. You merge with their essence which has been purified and is holy and you become pure and holy because that divine level of existence vibrates only with holiness, peace, and bliss.
By chanting a mantra, your cells, molecules, atoms, and sub-atomic particles all vibrate in the same wavelength as the mantra. Once attuned with this vibration you connect with everything resonating on that plane of existence. It's like tuning a radio. At first you may get static, but once you are in the right frequency your reception is perfect. Om is the universal sound. It is within every word and within everything.
What different types of teachers agree on is that mantras have the potential to take practitioners to the level of consciousness that transcends the limitations of the mind by a billion-fold. There is an ancient tale that very well shows that true devotion and complete absorption are the key:
Once upon a time in a land far away lived a poor uneducated, mentally challenged man who tended a herd of cows for his master. He happened upon a meditation teacher and was very taken with his calm, loving, gentle and happy nature. And so he went to the teacher and begged him to teach him a way to achieve the inner peace that radiated so obviously from the teacher. The teacher accepted him as his student but quickly found that the man couldn't understand any of the philosophical points he was making and as a matter of fact couldn't even remember the mantra Om when he tried to teach it to him. The teacher lovingly said, "My oh my, you don't seem to know anything at all, can't be taught, and can't remember anything. You are devoted and sincere in your desire to gain happiness though, so I will try to help you. My son, what do you know?" The man said, "Oh great teacher, the only thing I know is cows. All my life I've spent caring for cows, making sure they graze, are milked, and are kept clean. Yes, for me, everything is cows." "Well, that's alright," said the teacher, "then you know what sound the cows make." "Oh yes," said the man, "they say moo." "Very well then," said the teacher, "for you, moo will be your mantra. All you have to do is say moo continually and you will reach freedom from suffering and know real bliss." So the man chanted moo, moo, moo when he took the cows out to graze and he chanted moo, moo, moo when he milked them, and he chanted moo, moo, moo when he cleaned them. He chanted moo all the time and very soon merged with that vibration, which is Om backward, and reached the highest heights of joyous understanding and lived happily ever after.
From this story, we learn that it is virtually impossible to chant Om "wrong". The giver of this sound to the universe knows the intention and devotion of the practitioner and that is by what we will be rewarded. It is said that one minute of sincere chanting is superior to a thousand hours of mere sounding of the words. A parrot can be taught to recite a mantra but this will be just mere vibrations in the air. It is the love and worship behind the sound that counts. Technically though, there is a "correct" way to chant Om. It is made up of three letters: A, U, M. The A is guttural and comes from the throat. It is pronounced without any part of the tongue or palate in contact. The U sound comes from the middle of the sounding board, the palate. In Sanskrit, the A and U join together to become O. The O sound is vibrated from the navel/ solar plexus area and sent up to the sternum to the voice region, the lips, where the M sound is prolonged and vibrated up to the crown of the head. This vibrating M is felt in every cell of the body and is beamed out lovingly, soothingly, powerfully, to everything, everyone, everywhere. Intellectually and metaphysically, A stands for the physical world perceptible to the senses, the material world. U represents the astral and dream planes, heaven and hell. M is the unknown, deep sleep, and that which is unfathomable to the intellect.
Meditation Society of America
There are three ways to perform mantras: aloud, silently but while mouthing or humming the mantra, and completely silent within oneself. Chanting Om silently is considered the most potent method because it is not dependent on having a human voice box, lips, or facial muscles, all of which are temporary manifestations compared to the billions of years you will be fully at one with Om.
It is advantageous to your spiritual development to consider the theological, philosophical and mystical aspects of Om while chanting with your physical eyes closed, looking through the third eye, and paying attention to your breath. This may seem complex and complicated, but once in synch, it happens naturally as Om reveals itself, your Real Self, to you.