Is it recommended for the Imam to recite the Takbir aloud?
Chapter on the Description of Prayer
Al-Mughni
Book of Prayer
Primary text
It is recommended for the Imam to recite the Takbir aloud so that the followers (Ma'mumun) may hear it, as they are not permitted to recite the Takbir until after the Imam has done so. The evidence for this practice is the narration from Jabir, who stated that the Messenger of Allah (peace be upon him) led the prayer with Abu Bakr behind him, and when the Messenger of Allah recited the Takbir, Abu Bakr would recite the Takbir so that we might hear. This is reported in Al-Muttifaq 'Alayh (Agreed upon by Bukhari and Muslim).
Supporting text
If the Imam cannot make himself heard by the followers, then some of the followers must raise their voice in Takbir to make those behind them hear, or to ensure those who cannot hear the Imam can follow.