Do people who tote around thousands of sonically flattened, Pro Tooled songs in their iPods know that most of what they’re hearing is closer to a computer program than it is to music? Nowadays, pop music is mainly fast food to be gobbled on the go, to be heard through earbuds or on portable docks with plug-in speakers. As long as it sounds good enough, nobody seems to mind.

But Greg Milner does. A contributing editor for Spin and the co-author (with Joe Berlinger) of Metallica: This Monster Lives, Milner has explored a century-plus of efforts to capture something as ephemeral as a voice or