Unison notes are too loud due to matched phase

In the real world, different singers and different instruments in general have non-correlated phase. With N parts on the same instrument the overall sound power will be N times as high. So the amplitude is sqrt(N) times as high. Two parts playing at the same volume will be 3dB louder than the single player and four parts 6dB louder, whether they're playing the same note or different notes.

In Musescore, when N parts join in the same note, they have identical phase, so the amplitude is N times as high and the overall sound power is N^2 times as high. Four parts at the same volume sound 6dB louder than a single part when they're playing different notes, but when they come in unison, suddenly they're another 6dB louder than that.