You've stumbled on a Number Station, a radio program purportedly used by one of the world's intelligence agencies to send secret messages. These stations have been broadcasting since World War I.
These sounds have now been mixed into movies and music.
Listen to a short NPR radio program here>>
Download Number Station broadcasts from The Conet Project at the Internet Archive>>
More from The Straight Dope>>
And if you want a crazy amount of detail, including the numbers themselves, try The Spooks Newsletter>>