When davening, how much should I pronounce the words? Is it enough to, as the Rambam suggests, say them in my mind? Should I, like the Shulchan Aruch says, form them with my lips? Or do I need to at a minimum whisper them so I can hear?
1. I assume that your question relates, in the first place, to Tefillah, i.e. Shemoneh Esrei. Rambam does not suggest that one should recite the Tefillah silently, i.e. in one’s head. In Hilkhoth T’phila 5:10 Rambam states the opposite, namely that while one should not daven aloud, one must enunciate the words with one’s lips and whisper to oneself. Rambam goes on to say that if one is ill, or unable to concentrate without reciting the Tefillah out loud, one may do so in private, but not in a minyan so as not to disturb others. This is also the view of the Shulchan Aruch Orah Hayim 101:2 who quotes Rambam nearly word for word.
3. The difference between Tefillah and other brachot is that during Tefillah one should whisper, whereas other brachot should be said out loud. See Rambam's MT Hilchot Brachot 1:7.
28th of Sh'vat, 5773 Thursday, 07 February 2013