Just another random tidbit that many of us know but I think its important to note in particular:
Once you have completed the audio log collections the cutscene at the end of Data Hive changes. In the new ending is the only time we hear the Rookie vocalize in the entire game. Well, except for grunts of pain and sighs of relief.
The Rookie only ever speaks the language of the EngineersYou can view this and all the rest of the ODST cutscenes plus variants at HBO:
http://nikon.bungie.org/misc/cutscenes/odst.html