Birkat Kohanim: Singing
Article
There is an ancient and widespread custom for the Kohanim to chant a tune between each of the three verses of Birkat Kohanim, something that may even pre-date the Talmudic era. (1) It has been said that this tune is the one that was used in the Beit Hamikdash and may even date back to Mt. Sinai. The singing serves to separate each of the three verses of the blessing. This is because each verse is actually an independent blessing. (2) In some congregations, the singing takes place before the Kohanim recite the last word of each verse. Outside of Israel, where Birkat Kohanim is performed only on Yom Tov, the singing is often an elaborate and central feature of the service. In Israel, even though Birkat Kohanim is performed daily (in most of Israel) the singing is only done on holidays.
It is explained that the chanting inspires the kohanim and assists them in attaining higher levels of concentration and intention. (3) Indeed, song and music are a means for arousing the Divine presence to rest on a person. (4) Music also puts people in a happy mood and the Divine Presence only rests where there is joy and happiness. Indeed, the Kohanim must be in a good mood when giving the blessing. Singing was an integral part of the daily service in the Beit Hamikdash (5) and we are told that it helps in having prayer accepted. (6)
Nevertheless, there have been authorities who frowned upon the custom of singing during Birkat Kohanim. It is argued that the chanting forces the Kohanim to remain with their arms extended longer than required, which can be uncomfortable. There is also the concern that both the Kohanim and the congregation, could lose their focus and concentration during the periods of extended chanting.
Le'halacha it is permitted for the cohanim to sing while the tsibbur recite the Yehi Ra'tson. In some congregations, the kohanim don’t sing when Yom Tov and Shabbat coincide.
On a related note, there is some discussion on the permissibility of singing or humming a tune when interruptions are generally forbidden. For example, some people sing a tune after washing their hands before bread, while waiting for others to wash their hands. According to a number of authorities, singing a tune at this time is a forbidden interruption between washing one’s hands and eating the bread. Other authorities argue that a wordless tune is not considered to be an interruption and permit it even when interruptions are otherwise forbidden, So le'halcah one should refrain from doing so.
Source
2. Rema, OC 128:45.
3. Aruch Hashulchan, OC 128:35.
4. Melachim II 3:15.
5. Erchin 11a.
6. Baal Haturim, Devarim 3:23.