I came across an article on Al-Islam.org on the subject of Tawbah, where a tradition of the Prophet states that if one of his ummah follows certain A’mal for Tawbah (mentioned in detail on the link below), he or she will;

“Anyone from my Ummah (community) who does this will hear a voice from the sky that his repentance (Tawba) has been accepted and his/her sins pardoned. An angel in the Arsh (Throne) will say: “Those who were displeased with you (in the worldly life) will be pleased with you on the day of Qiyāma”. Another angel will announce: “O Believing Slave! Your grave will become wide and bright. Another angel will say: Your parents are pleased with you and, due to your performance, they too have entered the Mercy of Allah. Your provision in both this world and the Hereafter will be abundant. Hazrat Jibrīl (a.s.) says: At the moment of your death I will come to you along with Izrāel (the angel of death) and recommend to him to retrieve your soul with ease”
(Kitab al-Iqbal)
Link: https://www.al-islam.org/greater-sins-volume-3-ayatullah-sayyid-abdul-husayn-dastghaib-shirazi/tawba-part-one

My question is, is this “voice” an actual audible voice we can hear from the heavens? Since we aren’t prophets how can we hear divine Angels? And how is it possible that divine voices be ascertained in our physical world? Or is this hadith referring to when one is on their deathbed as the Angel of Death is coming to them?

Finally, a bit of an irrelevant question—but did Imam Ali pray behind any of the caliphs? For unity or any other purpose or did he strictly lead/pray alone?

1- The hadith has not been translated accurately. The Arabic text doesn’t say he will “hear.” Rather it says an angel will call out, or that it will be “announced.” In this world, there is a barrier that prevents us from seeing and hearing the angels. The Prophet (s) in this hadith is informing us that when we repent this is what happens, and we know he is truthful. So he is giving us news of the unseen world. Yes, when we die and the barrier is lifted, we can see all these realities.

2- It seems that sometimes he had to be present in the mosque during Jama’a prayers. But there is no historical evidence that he prayed Jama’a. One hadith in the book Tafsir al-Qummi states Imam Ali (a) stood behind Abu Bakr “and prayer by himself.” This means Imam Ali (a) would pray individually and not Jama’a