He was asked:

A man established an endowment and stipulated among its conditions that they should recite whatever they could of the Quran, glorify Allah, proclaim His oneness and greatness, and invoke blessings upon the Prophet, may Allah bless him and grant him peace, after the dawn prayer until sunrise. Is it preferable to do this quietly or aloud? And what if the founder makes a stipulation concerning this?

He answered:

Praise be to Allah. Rather, performing remembrance and supplication quietly—including invoking blessings upon the Prophet, may Allah bless him and grant him peace, and other supplications—is preferable. Nor is it absolutely preferable except where there is an overriding consideration; it is especially preferable at this time. For Allah says:

“Remember your Lord within yourself, humbly and with fear.”

An authentic report relates:

“When the Prophet, may Allah bless him and grant him peace, saw the Companions—may Allah be pleased with them—raising their voices in remembrance, he said: ‘O people, be gentle with yourselves! You are not calling upon one who is deaf or absent. Rather, you are calling upon One who hears and is near. Indeed, the One upon whom you call is nearer to each of you than the neck of his mount.’”

Another hadith states:

“The best remembrance is that performed quietly, and the best provision is that which suffices.”

And Allah knows best.