The Laws of Prayer (The morning services) (39)

1. One should not pray only in one’s mind. Instead, one must enunciate the words with one’s mouth and say it loud enough that he himself can hear it, but quiet enough that nobody else can hear it. There are those who say in the name of the Zohar that one must pray so quietly that even one’s own ears cannot hear it. (Shulchan Aruch and Mishne Berurah 101)

2. If when praying quietly he is unable to concentrate, if he is praying at home he may pray out loud in order to be able to concentrate, whereas if he is praying with other people he may not make any noise at all, so as not to disturb others.