Is it permissible for someone who has already performed the obligatory prayer to lead in prayer someone who has not yet performed it, in the Asr and Fajr prayers, and should he recite aloud if he leads them in an audible prayer?

It is permissible for someone who has performed an obligatory prayer to lead someone who has not yet performed it, even if the former has prayed a supererogatory prayer. This is permissible in all prayers, and one should recite aloud in prayers where vocal recitation is prescribed. The two Sheikhs (Bukhari and Muslim) reported from Jabir (may Allah be pleased with him) that Mu'adh used to pray 'Isha with the Messenger of Allah (peace be upon him) and then return to lead his people (in prayer). This is the view of Ash-Shafi'i and Ahmad in one of the two narrations, while Abu Hanifa and Malik held that it is not permissible. In a narration from Ahmad, it is stated that it is not permissible except out of necessity. It is preferable to avoid it except out of necessity, as this is more cautious.