“Prisoh al shema” A good summary is to be found in a shiur from the The Israel Koschitzky Virtual Beit Midrash. In short, “…. ten men who prayed alone can subsequently fulfill their obligations ...
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 ...