In Dishonored 2, Audio Logs and Diaries are collectible Objects, which you can find on the ship “Dreadful Whales” and reward you with the Achievement "Imperial Seal" when you have all found. They all refer to Meagan Foster and Anton Sokolov, who accompany you on your journey on the ship.
You visit the ship between the missions and it serves almost as a headquarters for Corvo and Emily. For the Achievement or the Trophy count only the Audio Logs and Diaries you find on the ship. Other of these collectible objects outside the ship are not relevant. This makes the search much easier since you can collect them all on the ship so quickly.
Dishonored 2: Finding Audio Logs and Diaries
In the game, you visit the Dreadful Whales 6 times during the story. More specifically at the start of the missions 2, 4, 5, 6, 8 and 9. You will then find other audio logs and diaries.
It does not matter if you are playing with Emily or Corvo, or if your chaos level is high or low. Your decisions in the story are not relevant. For example, you can rescue Dr. Hypatia in the third mission, then reach the ship and leave her own diaries and audio logs. However, only those of Foster and Sokolov are relevant for achievement.
In the game, there are unfortunately no options to see which diaries and audio logs you already have or which are missing. So careful search and take everything on the ship with you. If you collected all the audio loges and diaries on the Dreadful Whales, which are connected to Foster and Sokolov, you will get the achievement "Imperial Seal".
Dishonored 2: Locations of all Diaries and Audio Logs
The locations of all 45 diaries and audio logs on the Dreadful Whales can be found in the following video. The timecodes can be found under the video.
Mission # 2 – 0:05
Mission # 4 – 3:07
Mission # 5 – 4:38
Mission # 6 – 6:05
Mission # 8 – 7:11
Mission # 9 – 8:16
Dishonored 2: all decorative objects / souvenirs locations
Dishonored 2: Emily / Corvo - best skills, power and abilities
Dishonored 2: Emily's weapons, skills and abilities