Answer
The entire acts of Salaat, time, procedure, etc. are Tawqeefi (based on revelation). We simply adhere to the orders exactly as given. However, one of the wisdoms of performing Salaat audibly and non audibly is that in the earlier times of Islam, the times of Dhuhr and Asr were regarded as dangerous times. Hence, Rasulullah (Sallallaahu Alayhi Wasallam) would perform Dhuhr and Asr Salaats silently so as not to draw the attention of the disbelievers unto the Muslims.
Although these conditions of danger disappeared later on, the law applicable remained intact upto this day.
and Allah Ta’ala Knows Best
Mufti Muhammad Kadwa
FATWA DEPT.
CHECKED AND APPROVED: Mufti Ebrahim Desai