On March 10, 1876, Alexander Graham Bell spoke nine words that heralded our modern age of rapid long-distance communication. "Mr. Watson -- come here -- I want to see you." And then, as soon as he spoke the words, they were gone.

This was a time before recorded sound was common, and any momentous series of sound waves were preserved only in memory and text. When you think of Kennedy imploring Americans to ask what they can do for their country, you can hear his voice -- its cadences, its timbre -- in your head. But for the greatest spoken words of the 19th century (or any time preceding), we've got to use our imaginations. (This is, of course, why so many Americans were bewildered by Daniel Day Lewis's interpretation of Lincoln's voice, even though it was recreated with the greatest attention to historical detail.)

But there are sound recordings that survive from as far back as the 1860s, '70s, and '80s. It's just that, until very recently, they were unplayable. We no longer had the right tools, and even if we had, playing them would ruin the wax cylinders or fragile records upon which the sounds were stored. But over the past few years, physicists have developed tools for creating 3D scans of the old records and converting those scans into playable audio files. Last year, they released the oldest playable American recording, a series of sounds from an 1878 demonstration of sound-recording technology in St. Louis. In it, you can hear laughter, a song, and some counting, all spoken into the world by people of another century.