Likutei Sichos – Vol 35 p 192 ff Three different reasons for why to Daven silently: 1) To show faith in Hashem (He can hear even a whisper) 2) Out of fear and respect one can simply not raise his voice. 3) To facilitate a deep kavana requires quiescence. Page 194-5.

Likutei Sichos – Vol 35 p 192 ff The issue of the three opinions is: what is the most important aspect of Davening? Asking for things or Kavana? The three views are 1) Asking for things and the whispering is for secondary reasons, 2) Asking for things but the the whispering is needed for the…