Found all Audio Logs, alone or with another ODST.
It is safer if you just do it solo (because you can quit and save anytime and pick up later), but my co-op friend and I found them all, and then read that it doesn't save.
As true as that is, you can still find them all and unlock the achievement in co-op. But you must finish all the missions and then follow the security guard into his secret room for the last audio log. You CANNOT quit the game and reload a mission. We breezed through all the missions, since it was on easy.
So thankfully, he and I unlocked the achievement. Good luck everyone.
Halo: ODST 1000/1000