Question: When listening to chazaras hashatz I sometimes hear the chazzan say modim quietly while everyone else says modim derabanan. Is this correct?
Answer: The Gemara (Sota 40a) writes that when the chazzan reaches the beracha of modim, the tzibbur say modim derabanan (See Rambam, Tefilla 9:4). This beracha is so important that the Gemara (Berachos 21b) writes that one who comes late to shul should not begin davening the amida unless he knows that he will finish before the chazzan reaches modim (See Shulchan Aruch OC 109:1).
The Mishna Berura (124:41) questions the practice of some chazzanim who say modim quietly. The chazzan must raise his voice if necessary, to ensure that he can be heard by at least a minyan of men.
Thus, R’ Ephraim Greenblatt (Rivevos Ephraim 1:91:1) questions whether the tzibbur should say modim derabanan quietly so as to ensure that they can hear the chazzan. Elsewhere, (ibid. 2:185:17; 5:76) he quotes R’ Yosef Shalom Elyashiv as saying that the chazzan should pause while the tzibbur say modim derabanan before continuing modim to enable everyone to hear. One shouldn’t do this, however, unless it is the minhag of that shul.In conclusion, the chazzan should raise his voice a little while saying modim so that the tzibbur can hear him.