Is it permissible to raise one's voice with the takbir and tasmi' in congregational prayer, and what is the ruling on that, especially since the imam has forbidden it, and does that diminish the reward of the prayer?

It is desirable for the Imam to vocalize the opening takbir (Allahu Akbar) and the takbirs of transition so that the worshipers behind him can hear. As for the worshiper praying behind an Imam and the one praying alone, it is sunnah for them to say the takbir silently, and the minimum level of silence is for the worshiper to hear himself. Vocalizing the takbir for anyone other than the Imam is not from the Sunnah, unless there is a need and it does not lead to disturbing the worshipers. The Malikis hold that it is desirable to vocalize the opening takbir for every worshiper (whether Imam, worshiper behind an Imam, or one praying alone), whereas vocalizing takbirs other than the opening one is desirable only for the Imam.