QUESTION: We had an interesting discussion about the sacred syllabe OM used in so many religions (Hinduism, Jainism, Buddhism...) Someone said that in a mantra, OM could be replaced by any other sound since we are the ones who give a sacred power to the word which in itself means nothing. Some seem however to admit that the syllable has in itself a certain vibration which is beyond what we may attribute to the word. Also someone said that the only function of the recitation of a mantra is to pacify the mind and its endless discourse, so the words in themselves are of no importance. I would very much appreciate your comment on the matter.
SHORT ANSWER: There are many dimensions to this, not just one right one.
LONGER ANSWER: I don't think that any of these positions encompasses the whole matter - there is some justice in each but none is enough on its own. Words are not entirely arbitrary. Take the English word "fish". It is certainly suggestive of the movement of the animal through the water. It seems probable that music preceded speech in the sense that our most remote ancestors probably sang to each other a bit as birds do. Suggestive sounds would be at a premium in such communication. The part of the brain that deals with musical sound is in the right side of the brain. In the symetrical part of the left brain we store vocabulary. Gradually words and verbal vocabulary evolved and became more and more elaborate and nowadays much is relatively arbitrary, but that is a modern (i.e. in the last ten thousand years or so) development.
The specific sound OM or perhaps it might be better written Aum, is said to encapsulate all the sounds that humans make. If you say it slowly you will notice that your mouth starts off wide open and gradually closes as you go through the range of sound. This, therefore, is not a purely arbitrary syllable. "Gosh" or "Ouch" would not do the same job - though they do do other jobs quite well. Since Aum encapsulates all sounds it is the mother of speech. This is what gives it its special, divine quality.
Of course, it is the disease of our age to overrate our control, power and rationality. Nonetheless, intention does make a difference. In a phrase like Namo Amida Bu, the actual words are less crucial. It is the intention that they express that matters. In a different country you find different words doing the same job. No problem. So long as one intends what one says to be an invocation of Amitabha Buddha, any suitable words will do.
As for the function of a mantra, it is a protection for the mind. This functions in a number of ways. In the case of a mantra like OM there probably is a resonance with something deep within our archetypal psychology. Then, any mantra builds up and becomes a key to a host of good associations. The single syllable thus becomes a trigger linking one to a great spiritual wealth. The fact that the same words have been used from time immemorial also adds weight to this effect. One is connecting with the ancestors and their power. At the most simplistic level, while the mind is full of the mantra it is not full of other less wholesome things.
So I don't think that the answer to your question is just one simple point. There are many dimensions to what is at work. The mantra does its work upon us and we remain mostly unconscious of the effect. It keys into us at a deeper level than the merely rational. I imagine that it reaches something on the right side of the brain.