The Shulhan Aruch rules (Orah Haim 128:14; listen to audio recording for precise citation) that the Kohanim must recite Birkat Kohanim "Be’kol Ram" – in a loud voice. The Mishna Berura (commentary by Rav Yisrael Meir Kagan of Radin, 1839-1933) clarifies that this refers to a "Kol Benoni," an ordinary, audible voice. Generally speaking, the Kohanim are not required to shout the Beracha, but they must not recite it in a whisper. However, the Mishna Berura adds, Halacha does require the Kohanim to endeavor to ensure that the entire congregation hears their voices. Therefore, in a large congregation, the Kohanim must raise their voices and recite Birkat Kohanim loudly, such that everyone in the synagogue could hear the Beracha.
If a Kohen does not have a voice that carries to this extent, or if a Kohen suffers from a cold or from laryngitis such that he cannot project, may he recite Birkat Kohanim?
Hacham Ben Sion Abba Shaul (Israel, 1923-1998) rules that a Kohen may recite Birkat Kohanim so long as ten men can hear his voice. Even though the Kohen should preferably ensure that the entire congregation hears his voice, he may recite the Beracha if he can be heard by a Minyan, despite the fact that others in the synagogue will not hear the Beracha.
If, however, a Kohen is capable of speaking only in a whisper, then, as the Mishna Berura writes, he should not recite Birkat Kohanim. In such a case, the Kohen should leave the synagogue before the Hazzan recites "Reseh" and remain outside for Birkat Kohanim.
This Halacha should serve as an important reminder to Kohanim to project while reciting Birkat Kohanim, and not recite the Beracha softly.
Summary: Kohanim should preferably recite Birkat Kohanim loudly enough that they could be heard by the entire congregation; at very least, they must be heard by ten men. If a Kohen cannot project to the extent where he is heard by ten men, such as if he has a very soft voice or suffers from laryngitis, then he may not recite Birkat Kohanim.
