Mosques are houses of Allah on earth, so whoever enters it would observe its etiquettes, of which is the reduction of voices to our best. In this regard, Al-Bukhari related on the authority of As-Saib ibn Yazid, who said: (I was standing in the mosque and somebody threw a gravel at me. I looked and found that he was 'Umar bin Al-Khattab. He said to me, "Fetch those two men to me." When I did, he said to them, "Who are you? (Or) where do you come from?" They replied, "We are from Ta'if." 'Umar said, "Were you from this city (Medina) I would have punished you for raising your voices in the mosque of Allah's Apostle.)

The Prophet frequently warned against raising voices at mosques even during lining in congregational prayer where Imam Muslim related on the authority of `Abdullh ibn Mas`ud who said that the Prophet (PBUH) said: (those gifted with intellect should line behind me, then those lesser, and then those lesser. Beware of markets riot).

The Prophet forbade them from raising voices as prayer is a sort of presenting oneself before Almighty Allah which entails calmness and etiquettes of servility, and it is incontestable that reducing voices at mosques spread the spirit of tranquility and quietness which leads to submission in prayer.  

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