Is it necessary to audibly utter the Two Testimonies (Shahadatayn) to repent from apostasy, or is it permissible to utter them silently by moving the tongue and lips, as in prayer?

It is not a condition for the validity of entering Islam to pronounce the Two Testimonies aloud, and no scholar has stipulated that. As for the devotional recitations in prayer, such as the Takbir and the Quranic recitation, scholars have differed on whether it is a condition to make oneself hear them, or if merely moving the tongue and lips is sufficient.