When should one recite aloud in prayer, and when should one not, especially given that devotion (khushu') is only achieved through audible recitation?

The jurists have agreed upon the desirability of audibly reciting in the Fajr prayer, and the first two units (rak'ahs) of the Maghrib and Isha prayers, and the Jumu'ah (Friday) prayer, and silently reciting in the Dhuhr and Asr prayers. This was the practice of the Prophet, peace and blessings be upon him. As for the voluntary night prayers, audible recitation is recommended for them, and silent recitation for the voluntary daytime prayers. An exception to this is made for daytime voluntary prayers for which people gather and which are preceded by a sermon, such as the solar eclipse prayer, the prayer for rain (Istisqa'), and the two Eid prayers, in which case it is sunnah for the Imam to recite audibly. It is preferable to adhere to the Sunnah regarding audible and silent recitation. Reverence (khushu') is a heartfelt act that can be achieved whether the recitation is audible or silent, by bringing to mind the greatness of Allah.