Might just be me, but I think the track 'Further' from Far Cry 3 really suits the prologue. Especially the bits set in Sweden and Finland: I really got the feel of people fleeing north, alone, as their world collapses around them - and putting a brave face on it all the while.
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And on the other end of the emotional spectrum, 'Friends' by Miracle of Sound for our intrepid crew. The song's about the cast of Guardians of the Galaxy, but the central theme of a crew of friends standing up for each other fits SSSS quite nicely. Plus it just sounds like the sort of thing that would get put on the Cat-Tank's CD player as they drive off in the morning.