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: New Alexandria
Similar to the easter egg in Halo 3 and Halo ODST, the voice of this suicidal marine is very similar or the one in the other games. You encounter this easter egg pretty naturally if you are just running through the level. If you go to the area where the sniper is on the mulitplayer map Reflection and then down the corridor to the right, you will find him.