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.
To try this technique, take two large paper cups and punch a tiny hole in the center of the bottom of each with a sewing needle or similar. Take a piece (perhaps 100 feet [30 meters]) of non-stretchable thread or kite string and thread each end through each hole. Either knot or tape the string so it cannot go back through the hole when the string is stretched. Now with two people, have each one take one of the cups and spread apart until the string is tight. If one of you talks into one of the cups while the other listens, the second person should be able to hear what the first person says.
Here is why it works. 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.
This is not much different from how a telephone works, except that electric current replaces the string in a telephone. In an old-style telephone, the person speaking vibrates a metal diaphragm. The diaphragm's vibrations rapidly compress and uncompressed carbon granules, changing their resistance. A current passing through the granules is strengthened or lessened by the changing resistance. At the other end, the rapidly changing current runs through a speaker and causes its diaphragm to vibrate back and forth, so the second person hears the first person speaking. See How Telephones Work for details.
In a radio, the changing current from the microphone is used to modulate a radio transmitter. The modulations are sent through the air to a radio receiver, which turns the modulations into the vibrations of a speaker, and the second person hears the first person speaking.
In a compact disc or a cassette tape, the sound waves vibrate the microphone and the vibrations are recorded on the tape or CD. In the tape, they are stored as magnetic fields in iron oxide. On a CD the vibrations are digitized and then written onto the face of the CD. See How CDs Work and How Tape Recording Works for details.
Here are several interesting links: