'Uqbah ibne-'Amir Radiyallahu 'anhu narrates that he heard Rasullullah Sallallahu 'alaihi wasallam saying: The one who recites the Qur'an aloud, is like the one who gives sadaqah openly, and the one who recites it quietly, is like the one who gives sadaqah secretly.(Tirmidhi: 2919)
Note: It appears from this Hadith that the preference is for reciting
the Qur'an quietly. This is the situation when there is a fear of 'Riya
(lack of sincerity). If there is no fear of 'Riya' and there is no
danger of disturbing others then according to some other traditions it
is better to recite in a loud voice as it may become a source of
inspiration and encouragement for others. (Sharh Tibi)