As discussed in this question, there are two reasons generally given why we say "Boruch Shem K'Vod..." quietly when saying Shema: The Talmud (Pesachim 56a) says that Yaakov Avinu said it in response ...
In Havdalah, I noticed a virtually universal custom: when the one who is saying Havdalah reaches the Passuk of Layehudim Haytah Orah (ליהודים היתה אורה ושמחה...), those who are being Yotzei Havdalah ...
Why do we repeat out loud the verses of Avinu Malkeinu - from Hachzireinu B'Teshuva until Kosveinu B'Sefer Slicha U'Mechila? Why only these verses? Why not others?
Why do we say Boruch Shem Kevod Malchuso L'Olam Voed in Kriyas Shema in a whisper?