When should I pray aloud?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Dear brother, you should recite loudly in the following Raka'a:
- Both Raka'as of Fajr prayer.
- The two first Raka'as of Al Magrib prayer. (sunset prayer)
- The two first Raka'as of Isha prayer.
This is Sunnah for both the Imam and the person who is praying alone, whether praying the prayer in its fixed time or making up a missed one.

As for Zuhr and 'Asr prayers, there is no loud recitation in them. Imam Ahmad narrated in his Musnad, he said: "A group of thirty companions of the Prophet gathered and said: 'We all know the prayers where the Prophet used to recite loudly".
So, there is no disagreement over this issue….

Allah knows best.