請觀音經疏闡義鈔 3.61

3.61

經「應當暗誦」，謂柔聲默誦也。此呪既爾，上下例之。然諸經事儀對機各別，如熾盛光呪則令厲聲，此文則使暗誦，幸順佛語方契聖旨。

The sūtra's "one should recite silently" means chanting quietly in a soft voice. Since this dhāraṇī is like this, the preceding and following sections should be understood by analogy. However, the ritual procedures of the various sūtras differ according to capacity: just as the Tejobala Dhāraṇī orders one to chant in a loud voice, this text has one recite silently. Fortunately, by according with the Buddha's words, one corresponds with the holy intent.