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 ...
The Rema in OC 61(26) says the custom is to say the first possuk of the shema aloud. In this question Sh'ma aloud but amida quietly Gershon Gold said that Shema is said aloud in unison and msh210 ...
Shulchan Aruch Orach Chayim 61:4: We are accustomed to reading the first verse [of "Sh'ma"] in a loud voice in order to arouse the direction [of our thoughts]. SA OC 101:2: But he should not ...