The early "voice notes" that give us a window into the past

Princeton professor Thomas Levin has collected the world's only collection of 'voice letters' -- small records that could be recorded on-the-spot in the 1930s, 40s and 50s, and then sent through the mail to friends and loved ones. From passionate love notes, to messages home from soldiers, to tourist diaries from world travellers...these notes allowed many people to record their voices for the first time ever. Levin explains how he searches through online auctions and flea markets to uncover these ghostly voices from the past, and what we can learn from them.