The expression of Mantras
Literally mantras are "mind protectors", words and sounds that structure conscious experience in a way that insures inner growth and cultivation. These words, phrases, verses etc, have four levels of expression.
1) The first level (manifest) is vocalized.
2) The second level (subtle) is felt, but unvocalized. This level is reached by subvocalizing and internalizing the sound to the extent that it is felt in the heart. Thus, there is still a vibration, but it only involves a very subtle contribution from vocal cords and diaphragm. It is as if the heart is spontaneously hearing the sound instead of the sound being uttered.
3) The third level (causal) addresses the thought beyond the sound. Here we "think" the sound, but in such a way as to generate the same effect as if it were vocalized. This is not abstract mind thinking but the full experience of awareness without actually going through a physical expression to make the sound. Here we simply express the intent to express. It is similar to imaginine we are expressing, only this is so vivid the body actually feels the vibration. As this becomes easier and without resistance, the vibration becomes independent of any one sense, and thus can influence all senses. The tone can be thus seen, heard, felt, smelled, emoted, etc, because the cause of the vibration has been unlocked from attatchement to effect. Hence it can globalize, and vibrates our whole being.
4) The fourth level is called supreme or eternal. Every vibration when traced to its causal root can be substituted by that root in awareness. Then the roots can be traced to a central core of all intention. The core flows out into causality (or various motivations and intentions) and these out into more structured expresssion. The most empowered mantras activate this flow system from start to finish, usually tracing our proficiency backwards from vocal to subtle to causal to the core of causality. In terms of vibration this core resides in the root of manifestation, traditionally at the base of the spine in human experience, but also in every inner center of chakra, especially the heart.
The fourth level is where any vibration generates a response from something transcending all vibrating form, something at the root of all existence. It is to this aim that a large body of mantra teachings are focused. At the same time aims such as influencing manifestation directly are best attainment through this transcendent connection.
It is known in western occultism that the deeper or more fundamental the level of existence we access the more profound the effects. Accessing the supreme level does not, however, mean the other three are left behind. Quite the contrary, different people are comfortable expressing mantras in different ways, and for any expressed vibration to be well-rounded it needs to be be learned at all levels, so it can affect all levels. It is all too easy to block off other levels by exclusive focus on one.
This is even true of the supreme level, which may detatch us from the phenomenal world unless we strive to include that world within our radius of identification and expression. We don't have to vocalize mantras for them to work. But exclusively being lost in the eternal doesn't mean they will ground into the manifest. If they don't then we will have to go into trance to experience the benefits, which isn't practical at all.
The point is to gain maxiumum experience while awake and clear with a full spectrum of emotional and sensate (feeling) experience, so that all levels come into our being in unity. Practically speaking, mantras must be pronounced correctly with the right rhythm or their force is diluted. Vocalizing guarantees we eventually get pronouncing right because the effects are felt in the body. As these effects amplify, we can move to subvocalize and feel/hear the sound coming from the chest, half-thought, half-breath. When we feel the vibration amplify as an energy we can move to express the mantra purely as a series of thought-out intentions of vocalizing, each representing the sound elements of the mantra properly pronounced. As we sensitize to this eventually, the mantra induces an altered state where consciousness cycles to be aware of itself, which is also the level that links it to form, the level where consciousness attains form without losing its fundamental nature.
When a mantra attaints this level without losing continuity with the other ones, it becomes awakened, or rather we become awakened to it. Then it can be used for occult purposes, be those to access realization of divinity or some kind of magical manifesting, healing, attacking etc.
The simplest way to realize these four levels of expression is to take a simple one syllable seed mantra such as OM and work with it. The traditions say it takes about 2500000 repetitions until a person embodies the vibration and the mantra reveals its secrets, as it were. If the syllable takes five seconds or so to pronounce, that many reps would take a person chanting 1hr/day about 9.5 years to activate the syllable. This gives an idea. If you chant 8hrs a day, then the time would be a little over a year. The good news is that when you can go through such an activation for one mantra, the rest come easier.