In a congregational prayer, will follower will follow the voice (Takbeer) or movement of Imam. Moreover if Imam forgets to say Takbeer, for example, going from Ruku' to Sujud and inspite of followers reminding the Imam that he did not say Takbeer, at this point, will the follower stay in his original position or follow the Imam's movement.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is mandatory on a follower to follow the movements of an Imam (leader in prayer) whether he hears the Takbeer (saying Allahu Akbar) or not. Saying Takbeer loudly is Sunnah for an Imam. One should follow the Imam even if the Imam does not say the Takbeer loudly but pronounces it quietly.
Allah knows best.