When we recite the Shema on Yom Kippur, we read Baruch Shem Kivod aloud and in unison. These are words that the angels exclaim in heaven and during the year we recite them in a whisper. Yom Kippur, however, is different because it is the time when our sins are forgiven. As such we feel confident enough to recite out loud that which the angles exclaim on high.