What is the ruling on vocalizing the takbeers of transition, ihram, and taslim, or vocalizing some of them but not others? And are they vocalized in the regular supererogatory prayers, such as the two rak'ahs of Fajr and Dhuhr?

The Imam should vocalize the transitional takbirs, the opening takbir (takbirat al-ihram), and the taslim, as should the musammi' (the one who relays the Imam's takbirs), so that the congregants may follow him. As for the ma'mum (follower) and the munfarid (one praying alone), they should not vocalize any of these. The Malikis, however, hold that the ma'mum should vocalize the opening takbir and the taslim of exiting the prayer only. As for supererogatory prayers (nawafil) at night, vocalizing is preferred, whereas during the day, whispering is preferred, except that the Shafi'is hold whispering to be preferred in all regular Sunnah prayers.