The Conet Project - Recordings of Shortwave Numbers Stations (1997)
I've been reading up on numbers stations lately so I've decided to dive into this four disc collection of recordings. It makes for some weird and creepy listening, with background static, noises, and weird musical jingles adding to the creepiness. Some of these recordings have been sampled by the likes of Boards of Canada and Stereolab.
You can download it for free here -
https://archive.org/details/ird059