http://aruonline.blogspot.com
Member of the GameDays community:
http://www.gamedays.com/
I can confirm this as I had the same experience. I had found about 15 or 16 audio logs during the single player campaign, and then went back to specific levels to earn various achievements, which "reset" my campaign progress.
On Easy, I ran through Mombassa Streets using a map to find all of the Audio Logs, but found that none were available. At first, I thought that was because I had found all of the obvious ones during the campaign, but then I completed the first flashback mission and one or two additional Audio Logs became "retrievable."
Again, I ran through the entire city and could not retrieve all of the Audio Logs, so I completed the next flashback mission. After that, I found quite a few more, but five of them (not including the 30th on Data Hive) were still not "retrievable." That's when I searched on the availablity of Audio Logs and found this forum post. After completing the third flashback mission, the remaining Audio Logs were retrievable.