42. It is a mitzva to call the bracha out to the Kohanim word by word so that they do not get mixed up, as the posuk says, “אמור להם,” indicating the chazzan tells them what to say (תוס' ברכות דף ל''ד ד''ה לא יענה). See the Kli Yakar in parshas Naso, where he explains that the chazzan is the intermediary who brings down the initial flow of bracha from the Source of Brachos and pours them onto the heads of the Kohanim, who become vessels brimming with Hashem’s bracha. Then, the Kohanim pour them from their full vessel into an empty vessel when they bentch Klal Yisroel.
43. Call out loud. The one calling the bracha should say the words in a loud voice; there is no reason to say the words in an undertone (מאירי סוטה דף ל''ט, שו''ת שבט הלוי ח''ג סי' ט''ו, מו''ר בשו''ת שבט הקהתי ח''ב סי' נ''ח). On the contrary, calling out the brachos rouses the Kohanim to also say the brachos in a loud voice, powerfully. However, the one calling the brachos should not make his voice louder than the Kohanim’s voices.
