Examples of projects which make audible the usually unheard transmissions, provided by Andrew Austin...
David Rosenboom ("Brainwave Music") and Alvin Lucier ("Music For Solo
Performer") both did projects in the late 60's and early 70's using
Alpha-brainwaves to control sounds, or systems to create music.
Electronic Voice Phenomena.....in other words, voices (sometimes thought to
be of the dead) appearing in electronic emptiness, such as on blank
cassettes, or TV/radio static - see the following for more information:
http://touchshop.org/product_info.php?cPath=25&products_id=55
The Conet Project - an audio project focusing on shortwave radio "spy"
stations - covert transmissions intended for agents in the field. Very
popular during the Cold War, and now apparently re-appearing in Arabic
variants.....
http://www.irdial.com/conet.htm
You can download (legally, and for free) the content of the 4-CD set, and
it's accompanying booklet, here:
http://irdial.hyperreal.org/
VLF (Very Low Frequency) and ELF (Extremely Low Frequency) radio signals
(also known as "Natural Radio") - from natural and/or man-made phenomena
(both the U.S. and Russia experimented with ELF transmissions from
submarines during the Cold War).....Stephen P. McGreevey is one of the
authorities on this subject, he has his own site, and also has a 2-CD set,
and booklet, that can be downloaded freely....:
http://www-pw.physics.uiowa.edu/mcgreevy/
http://irdial.hyperreal.org/
Alan Lamb - Australian composer noted for his works where he contact-mic'ed
telegraph wires in the remote Australian outback, to produce music.....:
http://www.awrc.com/review/l/primal_image.html
http://www.sounddesign.unimelb.edu.au/web/biogs/P000277b.htm
There's also things like the (mostly) Japanese "Onkyo" movement - where
sounds are produced using equipment like mixing boards with no other devices
input into them (Nobekazu Takemura and Sachiko M are 2 names to seek out in
this area), and composers such as Rafael Toral, who have made works using
feedback loops from linking together effects pedals without any exterior
sounds (which, incidentally, harks back to David Tudor, and pieces like
"Rainforest" and "Microphone" which used similar tactics in the
1960's.....).
