Prayer: Should nāfilah prayers be recited silently or aloud?

Practical Laws of Islam as per the teachings of Imam Khamenei

Prayer: Should nāfilah prayers be recited silently or aloud?

English:

Question #713:

Should nāfilah prayers be recited loudly or quietly?

Answer #713:

It is mustaḥabb to perform the daytime nāfilahs quietly and the nightly ones loudly.

-Imam Khamenei, Practical Laws of Islam, Importance and Conditions of Prayer