- Tupolev
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- Honorable Member
It's a parallel story that ties everything together and takes the story from being a bunch of random events with some cool atmosphere to a meaningful and coherent whole. Unfortunately, Bungie didn't lay them out easier to the player, despite the fact that they're completely necessary for the storytelling.
Anyway, here's a transcript of the audio logs, and here's Bungie's neato online interface for viewing the logs themselves.
If you're interesting in why I'm talking about how they wrap stuff up into a nice bundle, this may get you started.