Numbers stations are clandestine shortwave radio transmissions that are used to anonymously convey one-way messages from spy organizations to operatives in the field. Most different broadcasts are thematically similar, with recordings of voices reciting numbers strung together in different ways. When decrypted with via a one-time pad algorithm, the resulting message is finally understood. To anyone else, it sounds like staticy nonsense and numbers that no one save the recipient will likely ever be able to decrypt.
The main menu audio for both The Conduit and Space Giraffe use a numbers station-like presentation. In the case of The Conduit, this is probably done to evoke a sense of the shadowy government plot at the heart of the game's story. In the case of Space Giraffe, it's probably just there because Jeff Minter is a weird guy.
A four-disc set of recorded numbers station transmissions was released as The Conet Project. The set can be legally acquired for free online.
In Call of Duty: Black Ops, the main character Alex Mason is repeatedly questioned about "the numbers", in reference to a numbers station broadcast heard throughout the game that becomes a focal point of the plot. The numbers also factor into flashback sequences in Black Ops II.