How should dhikr (mentioning the name of Allah) and dua” (prayer, supplication) be made after salah (prayer), Should it be performed loudly or silently?

Performing the dhikr and dua silently is sunnah. For, the dhikr and dua were not performed loudly during the era of bliss and four caliphs (al-Khulafa ar-Rashidin). However, if the congregation (jamaah) is ignorant, it should be performed loudly until it is taught, after it is taught, it should performed silently.

Since the new people of present time who do not know how to perform dhikr and dua join congregation (jamaah), it is much better that the dhikr and dua are performed loudly by the muezzin.