Audio Logs are little tape recorders that are hidden throughout the map and can be started by going to the folder icons on the map. You must track down the Audio Logs by using your phone, which indicates how far away from the objective you are. If you see the distance getting larger, then you know you are heading in the wrong direction, but note that sometimes height plays a part, too.
You know you’re getting closer to the Audio Log when you can hear some static noise. Once you’re close enough, you’ll see a prompt to use the Touch Pad to pick up the Audio Log. It’s not clear who the person is exactly who recorded these, but they provide a little bit of backstory for certain characters and events.