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The Sunnah is to say it aloud, as the Prophet (salallaahu 'alaihi wa sallam) used to say it aloud. Ibn 'Abbaas (radiallaahu 'anhumma) said: "They used to raise their voices when making dhikr after finishing the prayer in the time of the Prophet (salallaahu 'alaihi wa sallam)." And if all of the people raise their voices, they will not disturb each other. But they will disturb each other if some of them say it aloud and others say it quietly. There is no doubt that it will disturb the one who says it quietly, but if he were to raise his voice like them, there would be no disturbance. As for those who are making up what they missed, they have brought it upon themselves; if they had wished, they could have come early and it would not have disturbed them. And as I said, if all of the voices were mixed together, there would be no disturbance, even for those who are making up their prayers, just as you see now on Fridays; the people all recite the Noble Qur'an and they raise their voices but, in spite of this, the worshipper comes and prays and he does not experience any disturbance.

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It has been authentically reported in the Sahihain (two Sahihs) on the authority of Ibn 'Abbaas (radiallaahu 'anhumma) that in the time of the Messenger of Allaah (salallaahu 'alaihi wa sallam) , it was the practice to raise the voice when making dhikr after the obligatory prayers; he said: "I used to know if they had finished (praying) when I heard it." [1] This authentic hadith and others which carry the same meaning, such as the Hadith of Ibn az-Zubair, Al-Mughirah bin Shu'bah and others all prove the lawfulness of raising the voice when making dhikr after the people have finished the obligatory prayers, so that the people at the door of the mosque or those near to it will know that the prayer is over. But whoever finds people praying what they have missed of the obligatory prayer near to him, it is better for him to lower his voice a little so that he does not disturb them, according to other evidences. And in raising the voice when making dhikr after the obligatory prayers, there are many benefits, including demonstrating the singing of Allaah's praises for the blessing He has conferred on them through their performance of this great obligatory prayer, and it teaches the ignorant and reminds those who forget; and if it were not for this, the Sunnah would remain unknown to many people. And Allaah is the Granter of success.