But the greater distance between stage and audience that comes with a move to a Broadway house softens the edges of its exaggeration. And what was always wonderful about “Once,” its songs and its staging, has been magnified. In the meantime its appealing stars, Steve Kazee and Cristin Milioti, have only grown in presence and dimensionality. Who would have thought that this soft-spoken little musical would have found itself by raising its voice?