All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is desirable to pray two Rak’ahs (units of prayer) as a Sunnah (Prophetic tradition) after the Maghrib prayer, and this is a confirmed Sunnah. Nonetheless, some scholars are of the view that it is desirable to pray six Rak’ahs and they called it the prayer of neglectfulness. A Dha’eef (inauthentic) Hadeeth has been reported about these Rak’ahs. In any case, the prayer is a righteous act and the time between Maghrib and ‘Ishaa' is a time when it is desirable to perform unrestricted supererogatory prayers.
With regard to the recitation of Chapter Ash-Sharh [Quran 94], then we are not aware of any evidence that it has a relation with what you mentioned or with opening the breast of the Prophet . Indeed, the opening of his breast is among his great miracles and it is not like surgical operations.
As for raising the voice behind the Imaam, then it is permissible in case of necessity, because when the Prophet’s voice became weak, Abu Bakr used to repeat the Takbeer (saying Allaahu Akbar) after the Prophet for the people to hear it. Moreover, the Mu’ath-thin (caller to the prayer) in Al-Masjid Al-Haraam does so too for a need.
Allaah Knows best.