On 19b, the bach in Rashi S.V. "Bam" adds the word reshus and it seem a bit questionable to me why he did that -- is Rashi not saying that you have a chiyuv to say shema out loud as brought in shulchan aruch 63? So why does the bach add the word "reshus" there?
I think you mean Shulchan Aruch OC 62:3, based on Berachos 15a, "Hashme'a l'Oznecha Mah she'Atah Motzi mi'Picha."
It is true that one should hear the words of Keri'as Shema as he says them, but that cannot be what the Derasha of "Bam" is teaching us, for that is learned from the word "Shema" (Berachos ibid.). Besides, how do we see from "v'Dibarta" anything about hearing what one pronounces?
Rather, the word Bam is teaching a law involving Tefilah , that is, that it is not permitted to have others hear what you say during Tefilah. (The source brought for this Halachah in Berachos bottom of 31a is only from Navi, and not from Chumash.) This is what Rashi means (as is clear from Tosfos Yeshanim); he does not say that during Shema one may be "Mashmia l'Oznav , but that one may be Mashmia, that is, to others (as we find in Shulchan Aruch OC 61:26). This Halachah is learned from the word "v'Dibarta," since the word Dibur is normally used to refer to speech when it is addressed to another person (v'Dibarta El ...), as opposed to Amirah.