You can begin out loud, but you must learn to mantralize mentally, in silence, for the reasons you just mentioned, but also since mantras are more power in silence than in verb, as Swami Sivananda explained.
Joyful in hope, suffering in tribulation, be thou constant in thy prayer.
Benedictis, qui venit in nomine Domini. Osanna in excelsis.
"Blessed is he who comes in the name of the Lord. Hosanna in the highest!"