What is the reason for the need to be silent during Asr and Dhur?
I once heard in a speech that it's because Dhuhr and Asr are during the day when there are people circulating and at the time of prophet Muhammad (saw), praying out loud during the day could have brought attacks on the Muslims so prophet Muhammad (saw) would pray them silently and he (saw) would pray Fajr, Maghrib, and Isha out loud since they are prayed during the "quiet" times of the day.
I hope that I made sense.
edit---
Found this:
“The wisdom behind making Zuhr and `Asr silent, may be traced to the situation of the early Muslims in Makkah. During that period, Muslims were subject to persecution and torture. Thus, they were commanded to make their prayers silent in order to evade the torturing of the disbelievers who used to harm them. At the early morning, when the whole Makkah was sleeping, Muslims could make their prayer loud. Also, after sunset, the Quraish used to be busy with their social gatherings or celebrations where they used to drink wine or the like. So that time also was suitable for Muslims to make their prayer loud.
It is thus clear that silent recitation in both Zuhr and `Asr was just to evade the harm of the unbelievers. After their migration to Madinah, Muslims were no more subject to harm. However, the ruling of silent recitation for both Zuhr and `Asr was kept in force as a reminder for all Muslims that one day they were offering silent prayer out of fear of persecution. This also serves as an encouragement for all Muslims to praise Allah for His everlasting blessing and the overwhelming victory He granted them. Almighty Allah gives us a clear reminder of His grace in this respect when saying: “ And remember, when ye were few and reckoned feeble in the land, and were in fear lest men should extirpate you, how He gave you refuge, and strengthened you with His help, and made provision of good things for you, that haply ye might be thankful.” (Al-Anfal: 26)”
http://www.islamonline.net/servlet/...h-Ask_Scholar/FatwaE/FatwaE&cid=1119503545926