There have been a few interesting references lately to the "numbers stations," those shortwave radio stations that broadcast series of numbers, letter, or even musical tones. It is widely assumed that these are set up for intelligence agencies to communicate with agents in the field, though many other explanations are possible. An independent record label called Irdial in Great Britain has released 4 cds of these recordings under an open license. They are free to download and use for nonprofit purposes. I find them mysteriously compelling.
the home of the Conet Project at Irdial
Mirror of all 4 numbers station cds (If you go up to the parent directory you can find all the cds released openly by Irdial. I haven't tried any of them as yet. If you do, let us know what they're like.)
boing boing story on Irdial's lawsuit against Wilco