When we say Shema Yisrael, HaShem Elokeinu HaShem Echad, we acknowledge that everything out there has a singular source -- It's all HaShem.
When we say Baruch Shem Kvod Malchuto L'Olam Va'ed, we are saying that we are happy with everything HaShem has done.
So why do we say it quietly? (someone in the class asked.)
Because we can't always honestly say we're happy with every single thing that has happened right now (even when you lost a loved one?) -- we know and trust that really everything is good, but our current limited understanding requires us to say it quietly.
(In the world to come we will see how everything is really good, and then we will say it aloud.)