The old "two cans and a string" technique (or better yet, "two paper cups and a string") really does work. The key is to make sure that the string is tight between the two cups, and this normally means that the distance is limited and the two people have to be connected to each other by a straight line. But as long as the string is tight, it works. It is also a great way to understand how telephones and radio work.
When one person talks into his/her cup, the bottom of the cup vibrates back and forth with the sound waves. Imagine the bottom of the cup moving back and forth very quickly (1,000 times per second or more) with the sound waves of the speaker's voice. The vibrations travel through the string by pulling the string back and forth. Therefore, the bottom of the second cup should start to vibrate back and forth just like the bottom of the first cup is vibrating, producing sound waves. The second person can hear the sound waves and can therefore hear what the first person says.