Check out The Conet Project (
http://www.irdial.com/conet.htm ) a 4-CD box set of Number Stations recordings from the last 20 odd years(with an very informative booklet). Extremely creepy yet fascinating listening. To be honest, you're not going to sit down and listen to this after a hard day at the swamp, but it's a unique recording, and just
who is transmitting them??
I believe the sample that BOC used on 'Gyroscope' was a short wave radio recording made by Sean Booth of Autechre.