A music critic’s soundscape has been reshaped by the wail of ambulances. But she’s learning to hear in unexpected ways. --- Before the virus, there had been so much stimulus that many of us had learned to filter it out of our awareness — subway buskers’ pleas; sudden eruptions of earth-rumbling subwoofers at red lights — in order to preserve the emotional energy required to move through our days. But now in the absence of other sounds like heavy traffic, construction and the springtime shrieks of children on playgrounds, the sirens are all there is to hear. And of course, we cannot turn a deaf ear to what we know their escalating numbers signify.