O Shaikh, is it permissible for the one being led in prayer to say the takbiraat (takbirs), sami’a Allahu liman hamidahu, rabbanaa wa lakal hamd, and the tasleem aloud (i.e. in a voice that can be heard by others)?
It is not permissible for the one being led in the prayer to say any of the aforementioned things aloud. They are to only say them in a very low voice that could be only heard by themselves. They are not to say these things aloud as by doing so, they may disturb others who are praying. The Prophet (sallAllaahu alaihi wasallam) has prohibited us from disturbing others with thikr; meaning reciting the Qur’an very loud, the like of what is mentioned in this question as well as saying the thikr after the prayer in a very loud voice. However, the Imaam is to say all of the things mentioned in the question in a voice that could be heard by those praying behind him, and Allah knows best.
 i.e. Saying “Allahu Akbar”