Silent recitation is said to be the most powerful, since it requires the most concentration, attention, and remembrance of what one is doing at all times.

When you recite mantras internally, you are doing so within the mental and astral bodies, the internal vehicles, on a more subtle level.

It is also good to vocalize aloud to stimulate the glands, chakras, bloodstream, and organs, since this forces even Malkuth, the Kingdom or amalgamation of forces from above, to circulate and flow here and now, so as to promote internal work and penetration with our insight.