והכהנים והעם העומדים בעזרה
“And when the Kohanim and the people standing in the Temple court”

QUESTION: Why is it customary that the congregation sing along when the chazan recites this passage?

ANSWER: During the Yom Kippur service the Kohen Gadol would recite the Ineffable Holy Four Letter Name (The Tetragramaton) ten times. He would recite it very loud and it would be heard at a distance. However, once the “indiscreet ones” became numerous, he would intone it in a low voice so that they would not learn the proper pronunciation of the Name and use it improperly. Rabbi Tarfon, who was a Kohen in the times of the Beit Hamikdash said, “I was standing among my brothers the Kohanim, and I attuned my ear to hear the Kohen Gadol, and I heard him drown it in into the pleasant singing of the Kohanim.”

To commemorate this, the community sings so that the voice of the chazan will be over toned by their voices.

(ירושלמי יומא פ"ג הל"ג, רמב"ם עביוכ"פ ב:ו, ועי' ספר מקראי קודש)