The note says: Listen to the void... It tells you secrets.
Void?
There's that tank at location [-2 3]. You can yell at the void.
Okay, but what is the passcode?
The echo will tell you. Listen closely. You have to have sound on and listen what the echo says.
It is just singing
Listen to the "la"s in the answer.
Explain it to me
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.