What are the practical implications/applications today of the halachic principle Trei Kalli Lo Mishtamai (two voices cannot be heard simultaneously), aside from those related to Kol Isha? For example, if one is trying to say the Shema or the amida loud enough to hear oneself, is the hearing nullified if others nearby are saying the same thing at the same time? Why or why not?
Some accepted implications are laining torah, haftorah, megillat esther, and hearing shofar (should be only one). I am asking about possible other places that trei kalli could possibly be invoked.
In addition to the shema and amida examples above, there are various places that davening often (typically?) has communal singing or recitation in shul. Are these places exceptions, or marred due to the communal participation (should people refrain from singing along?)
In short, When does Trei kalli apply and when not?