Mystery of Number Stations: How spies communicate?

Let's start with the definition of Number Station. What does that mean? It's a type of shortwave broadcast and anonymous one way communication method, encrypted usually with numbers. They also can be transmitted in many different languages. Strings of numbers or phonetic letters can also be used.

Things got even creepier for me when I heard that little german girl's voice with a sound from a mucic box. Why?! (Listen to Swedish Rhapsody in the given link to see what I'm talking about)

To read more about it and listen them, click the link below. They have a bag filled with creepy number station records:

https://archive.org/details/ird059

If you find any creepy station record, leave it in the comments section.