During the '60s, a war raged in Vietnam. America's involvement caused many young men and women to be far from their families and in a very dangerous place. Communicating with family was next to impossible. Because of Vietnam's proximity to Japan, where most reel-to-reel decks were made, soldiers could get a reel-to-reel deck and tapes on the cheap. They would use these decks to record audio letters to mom and dad or maybe that special lady. Can you imagine? Your son is off thousands of miles away in the most dangerous place in the world; you're sitting at the kitchen table, you push a button, and there's his voice, "Hi mom, hi dad, I'm okay." Amazing! Perhaps not a time machine, but a cure for the worried soul. Those tapes still exist today, hiding in worn old boxes and plastic tubs, waiting to be heard again, ready to be that fantastic time machine.
Also of interest
Vietnam Tapes - Lost and Found Sound -- Recordings of Lance Corporal Michael Baronowski NPR
Vietnam-War Audio Correspondence - Smithsonian