Here is an amusing little snippet of film that demonstrates how sound effects were created for radio. It is an excerpt from a 1939 film called Back of the Mike that was produced by the Jim Handy Organization for the Chevrolet Motor Company.
If knowing how these sounds were created ruins the magic for you, then don't watch. Otherwise, enjoy this interesting, and chuckleworthy, look at radio actors and sound technicians.
To watch the complete version of Back of the Mike, click here.