What is the ruling on vocalizing during the last two rak'ahs of the 'Isha prayer, and is re-performing the prayer obligatory for one who does so?

Reciting aloud in the last two rak'ahs of Ishaa' or the last rak'ah of Maghrib, or in silent prayers, is not prescribed, but it does not invalidate the prayer nor does it necessitate Sujud As-Sahw (prostration of forgetfulness). Rather, it is makrooh (disliked). This is the madhhab of the Shafi'is, Al-Awza'i, and Ahmad in the more authentic of the two narrations, differing from the Malikis, Ath-Thawri, Abu Hanifa, and Ishaq, who hold that Sujud As-Sahw is obligatory. Al-Nawawi cited the hadith of Abu Qatadah: "And he would sometimes let us hear a verse." Ibn Qudamah held that reciting aloud and silently in their respective places are among the sunan (recommended practices) of prayer, and prayer is not invalidated by intentionally abandoning them. This applies to actions done by mistake or forgetfulness. As for intentional acts, the majority of jurists hold that prayer is not invalidated by them, while the Malikis, in one of their opinions, hold that intentional acts invalidate the prayer.