Say it now, say it loud! Mishna Berrua 619:2

The second line of the "Shm'a," the basic Jewish declaration of faith, is said in a whisper 364 days of the year. On Yom Kippur, we say it aloud. The Chofetz Chaim, author of the Mishna Berrua, explains why.

Shulchan Aruch: On the evening of Yom Kippur and through the next day, we say 'Baruch Shem Cavod Machulto' in a loud voice.

Mishna Berrua: This is because 'Baruch Shem' is the angels' song, and on Yom Kippur the people Israel are like angels.