Mantras are most powerful from within, since they require deeper attention, concentration, and presence. However, vocalizing mantras aloud, such as with the runes, charges our physical glands with tremendous force. Personally, I like to work with both in combination, since by working from within to without, and without to within, we become well-rounded spiritually.
Any mantra can be pronounced aloud, in whisper, or in silence. Mantras in alchemy can be performed aloud with great effect, or if you must work in silence, that will aid you too.
Something is created when we speak. Something is gestated. Combined with sexual transmutation, we develop the uterus or throat of the gods, which bear forth the ineffable Word.
For thirty years I sought God. But when I looked carefully I found that in reality God was the seeker and I the sought. -Bayazid al-Bastami