last audiohile in halo 3 odst?

[Yenz]

hi, i can't get to the room where the last audiophile in Halo 3 odst is. because the cop guy who opens the door (i watched an online video) just dies at some point in the mission for me. have i done something wrong? why can't i get it?

 

[Grey]

You first have to get all 29 audiophiles on normal difficultly. You also have to play the whole game. (all the levels) before you go down under the city. then the cop guy doesn't die.and shows were the last audiophile is #30.