I’m revisiting this thread since I’m going to refer to it in a newer one.

To respond to some of the above points: there is NO tension between GIRM 86 and 87. Only the anglophone Catholics would have that impression, for it is only the English translation’s error that creates it. Anyone reading GIRM in Spanish, French, German, Latin, or Italian would understand these two sections as being in harmony because “vocal unity” characterizes choral singing perfectly well.

I’ve recently written to my bishop to propose that the word “their” be removed from this paragraph of GIRM’s English translation. I encourage other interested folks to do likewise.