What is the legal limit for raising one's voice during the takbirat al-ihram, and must it be repeated if there is doubt about not having uttered it?

The majority of scholars hold that it is desirable for the Imam alone to pronounce the takbirat al-ihram (opening takbir) audibly. However, the Malikis consider it commendable for every worshiper to pronounce it audibly. As for takbirs other than the takbirat al-ihram, they are to be said silently by anyone other than the Imam, unless the congregants cannot hear the Imam's takbir, in which case it is desirable for some of the congregants to raise their voices to make it audible to them. The madhhabs have differed in determining the minimum level of audibility.