This is a secret song in the Nuketown Zombies map that can be played during gameplay.
Follow the steps below to activate it.

1) Go to the wrecked bus in the middle of the map.
2) Look on the chairs and you will see a teddy bear.
3) Aim at the teddy bear and press X.
4) You will hear a unique sound.
5) Go the the back garden of the house with the Nuketown sign outside of it.
6) Turn left at the shed in the garden.
7) You will see a split in the fence, look through it and another teddy bear can be seen.
8) Aim at the teddy bear and press X.
9) You will hear a unique sound.
10) Go to the yellow house without the sign.
11) Go upstairs into the room with a bunk-bed.
12) Look on the top bunk bed and you will find a third teddy bear.
13)Aim at the bear and press X.
14) You will hear a unique sound.
15) The secret song will be activated.

Here is a video tutorial: