Agatha Christie: And Then There Were None Cheats

Depending on whether you save certain people during the course of the game, you'll receive one of 4 different endings.

Once the safe has been opened, inside is a small drawer with slots with the letters A, E (3), H, L, R, S, T, U, V and W. Notice the poster above the radio just outside the safe. You will find the answer there in a song that Patrick sings. The answer to open this second hidden door is: Rule the Waves.