Here is an interesting comparison of Cortana over the course of the 4 halo games:
PTesh adds his dulcet prepubescent tones over the top of one of the new trailers for weapons...
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Level: Uplift Reserve
This is a nice little easter egg in the sense that it's somewhat of a tribute to an egg that was in Halo 3. On uplift reserve there are two marines having a conversation, one going crazy and suicidal, ranting about being trapped in a zoo by the covenant. The neat thing about this one is that the marines voice is the same as the marine in Halo 3 who is suicidal over the flood.