Challenge description

Thank god! I knew that I can count on you!

We have no time to spare. My boss is in his office. I work in the adjacent room with my colleague … and four shelves. I attached some audio, recorded moments ago. Help me getting the numeric code for the safe of my boss. I can hear it but I can't decode it. My colleague watches me!

Serra.

Flag format: {full-code-to-open-the-safe}