When we walk through ancient monuments, the silence strikes us: we simply cannot imagine what those who lived millennia before would have heard. But, to our Stone Age ancestors, dwelling in a quieter time, sound mattered much more than it does today. They had an acute awareness of rhythm and resonance, sang and played musical instruments, and ascribed magical qualities to many sounds. Exciting research--known as acoustic archaeology--has reconstructed this vanished aspect of long ago, allowing us to "hear" it again. Computer modeling and sophisticated equipment have calculated frequencies and timbres, demonstrating that stone-built chambers, sanctuaries, and even caves were deliberately constructed to enhance ritual sounds. This new knowledge both exposes the origins of music and reveals a lost world where echoes were seen as the voices of the spirits. Travel from chambered mounds in Ireland to French and Spanish Paleolithic caves to Mayan temples in Central America, and listen to the past once more.