Everything in the universe has its own sound and vibration. When you chant a mantra during meditation, the focus of your meditation is to merge with the vibration of the sound you are chanting and become one with that energy.
This results in you being at one with everyone in the world chanting that same mantra, and also one with everyone who has ever chanted it. You merge with the essence of others who have chanted that mantra and you connect with them on a spiritual level to become pure, holy, and filled with bliss.
Legend tells that once upon a time there was a poor man who was charged with tending a herd of cows for his master. The man, uneducated and mentally challenged, one day met a teacher of meditation and he was impressed by his calm, loving and happy nature.
So, he asked the teacher to show him how he could achieve the same radiating peace that the teacher so obviously enjoyed. The teacher quickly discovered that the man was unable to understand any of the points of philosophy he was trying to teach him, and he couldn't even remember the "Om" mantra despite going through it time and again.
The patient teacher said to the man, "You are certainly devoted in your desire to be happy, but you don't seem to know very much and have trouble learning, and you can't remember anything. Tell me, what is it that you do know about?" The man answered enthusiastically, "Teacher, the only thing I know anything about is cows.
For my entire life, I've cared for cows, kept them happy, fed them, made sure they are milked regularly, and kept them clean. My entire life revolves around cows." The teacher smiled and said, "That's fine, my son. Then you know what sound a cow makes." "Of course," said the man. "They say moo." "Of course they do," said the teacher.
"And since you know that sound well, that will serve as your mantra. All you must do is continue to say moo and you will eventually find that you no longer suffer and you are filled with bliss."
From that moment on, every time the man took the cows to the pasture to graze, he chanted "moo." While he milked the cows, he chanted, "moo, moo, moo." Before long, he was able to internalize the vibration of his mantra, and he became joyful and lived happily ever after.
This story is an excellent example of how it is virtually impossible to chant "om" wrong. The sound "om" is simply a sound, just vibrations in the air. It is the love, prayer, and feeling of worship behind the vibration that results in the chant bringing peace and joy.
So, although there is a technically correct way to pronounce the word "om," the actual word is not as vital as the meaning behind it. Whether chanting aloud, silently by mouthing the word, or totally silently within yourself, it is energizing, healing, cleansing, and calming.