Note: This game is also titled Forbidden Siren.
Find Archive 087 in Kyoya Sudo's stage (03:00 on Day 3).
Successfully complete the first two objectives on all levels in every scenario to unlock the first ending sequence. Then, press Square at the scenario selection screen.
Successfully complete the game after finding all 100 Archives. A bonus FMV sequence will be unlocked at the top right corner of the scenario selection screen.
To unlock Mission Objectives 2 for every character and/or scenario, complete 19 stages in any order. Then, press L1 + R1 while the stage is highlighted at the scenario selection screen. Note: You will automatically obtain it after completing the random 19 stages.