The gemorah in Sukkah daf 51b explains that, in the enormous shul in Alexandria, in order for everyone to know when to answer Amen when the shliach tzibbur (prayer leader, aka sha"tz) was davening, ...
Can one whisper t'fila (prayer)? I'm thinking of obligatory t'filos like sh'mone esre and "Sh'ma". I know one can speak t'fila quietly, even very quietly; in fact, sh'mone esre is said quietly. I'm ...