The small room in Teleporter B, there's a table with a book on it that says "I'M".

You'll have to find three more books scattered around the map.

One book you can find in Teleporter B on top of a barrel.

Another book can be found near the Wunderfizz machine, on top of a group of barrels.

And the last book can be found in Teleporter C, on a nearby shelf.

Go back to the room with the first book in Teleporter B, and place down the books on the table. You'll now have to collect souls. (About 25 I think.)

You'll know when its done if the purple souls stop going towards the books.

After you're done collecting the souls, you can press F on the table to hear the second easter egg song.