It is legislated for the Imam to raise his voice each time he pronounces Takbir, so that those praying behind him can hear him. As for the Ma'mums, it is not legislated for them to raise their voices during any of the Takbirs. They should utter it only to themselves. The raising of the voice by the Ma'mums is even considered a Bid`ah (innovation in religion), which is prohibited by the saying of the Prophet (peace be upon him):
Anyone who introduces anything into this matter of ours (Islam) that is not part of it will have it rejected.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.