To follow up on the "numbers station" mentioned at >>285242443 :
Looking through the game files, the sound files for the numbers and letters are stored in individual sound clips, see pic related - There is no individual sound clip for the either sequence. The encryption key and any resulting ciphertext are dynamically generated, maybe even unique for each new game. I don't have time to do another playthrough yet, but will take a look come the weekend. If anyone is keen to crack this themselves, record yourself near the (correctly tuned) radio at the start and in the lobby of [spoiler]the northern star[/spoiler] on the same playthrough. I imagine this should be a trivial transposition cipher so it should be easy to crack.