An audio letter is an audio recording

Sound recording and reproduction

Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording...

 sent by postal mail

Mail

Mail, or post, is a system for transporting letters and other tangible objects: written documents, typically enclosed in envelopes, and also small packages are delivered to destinations around the world. Anything sent through the postal system is called mail or post.In principle, a postal service...

 to a recipient. The practice was more common when charges for long-distance telephone services were higher and before the widespread use of the Internet

Internet

The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

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Discovered audio recordings offer a unique perspective into that time period and have often been used to present historical context in modern day radio and television programs.

Media

Media used in recording audio letters include:

• Wax cylinder
  Phonograph cylinder
  Phonograph cylinders were the earliest commercial medium for recording and reproducing sound. Commonly known simply as "records" in their era of greatest popularity , these cylinder shaped objects had an audio recording engraved on the outside surface which could be reproduced when the cylinder was...
  
  
• Reel to reel
  Reel-to-reel audio tape recording
  Reel-to-reel, open reel tape recording is the form of magnetic tape audio recording in which the recording medium is held on a reel, rather than being securely contained within a cassette....
  
  
• Cassette tape