.. Keypad passcode

Look closely the walls. There should be a hint note. ( Look at the wall in location [-3 2]. )

The note says: Listen to the void... It tells you secrets.

There's that tank at location [-2 3]. You can yell at the void.

The echo will tell you. Listen closely. You have to have sound on and listen what the echo says.

Listen to the "la"s in the answer.

The echo answers three verses (for example: "la-la-laa"). Each verse is one number in the passcode. Count the "la"s per verse to come up with the passcode. The order is important.

For example a echo "la-laa", "la-la-laa", "laa-laa-laa-laa" would become passcode 2,3,4.