The mission was a failure. You were supposed to confiscate a massive bomb but it had been destroyed before you could reach it. It had detonated and sent you all flying, destroying the Jeeps. Which is why all of the soldiers including you are in a big cargo plane on your way back to base.

Everyone was feeling, down and Soap decided to do something. He started singing.

"Puff out your chest. Take a deep breath. You're gonna be okay. Is it loud in your mind? Just take your time. You're gonna be okaaay."

After the first verse, more soldiers started singing. Your teammate Soap was looking at you, Captain Price and Ghost with a smile, all while singing.