Nichiren states, “The voice does the Buddha’s work” (Gosho Zenshu, p. 708). To chant the Mystic Law is to praise the Gohonzon. Hearing the sound of our daimoku, the heavenly deities will be set into motion and work to protect us. A weak and unclear voice will not move the heavenly deities. That is why it is important for us to chant daimoku with voices that are clear, strong and brimming with joy.
from sgi.org