Audio log (Halo 3: ODST)

Audio Logs are set to be in Halo 3: ODST. Nothing is known about them, but speculation seems to be that they are logs left by ODST's, Marines, or civilians retelling the events concerning what happened to the city, similar to the audio diaries from BioShock.

There are four [http://halo.wikia.com/wiki/Halopedia:News/24_June_2009/Halo_3:_ODST_Achievements_Discovered! Halo 3: ODST Achievements] that require you to uncover an particular amount of Audio Logs in order to be unlocked.