Why is the additional phrase "bruch Shem Cavod Malcuto Layolam Vaed, a non Torah addition, added to the chanting of Shema before the chanting of the vahavta?
The ברוך שם was originally the liturgical response to the recitation of Hashem's name in the temple. Of course, since we no longer have the temple, it is not used in that way.
We could consider that the Shma is our proclaiming of Hashem's Glory and uniqueness which would be similar (though not the same) as the use of the Name in the temple. That is, just as it was the recognition of the glory and presence of Hashem that caused the response in the temple, so to it is the recognition of Hashem's presence that cause us to respond (as did Yaakov when his children said the Shma).
There is a medrash in Pesachim 56, that when Yaakov avinu was not allowed to tell about the mashiach on his death bed. When his children said the Shma, he burst out with ברוך שם
The Midrash (Devarim Raba 2:25) relates that we do not say the praise "Baruch Shem..." out loud because Moshe Rabbeinu "stole" it from the angels whom he heard saying it when he went to heaven to receive the Torah. We "hide" it from the angels by saying it in a whisper. However on Yom Kippur, when we are raised to the level of the mal'achim, we can make a public declaration just as Yaakov Avinu did.
Reciting "Baruch Shem Kevod Malchuto" Silently explains that we normally recite it silently because it is not in the Torah and also that we are not on a level that would allow us to emulate the mal'achim.
Yaakov was overjoyed to hear his sons' collective declaration, and he praised G-d by exclaiming, "Baruch Shem Kevod Malchuto Le'olam Va'ed" ("Blessed is the Name of the Glory of His Kingship, forever!"). In the Book of Devarim, however, when Moshe declared the verse of "Shema Yisrael," he did not then proclaim "Baruch Shem." We thus face the dilemma of whether we should follow Yaakov's example, and recite "Baruch Shem" after "Shema Yisrael," or the precedent set by Moshe, to recite "Shema Yisrael" without adding "Baruch Shem." For this reason, the Gemara writes, we recite "Baruch Shem" in a low tone after reciting the verse of "Shema Yisrael." We do this as a compromise, of sorts, between the respective practices of Yaakov and Moshe.
(The one exception is Yom Kippur, when we declare "Baruch Shem" in a loud tone; we explained the reason for this practice in an earlier edition of Daily Halacha.)