When you first start the game, you will be on the Virtuous Mission, where you have to rescue Sokolov. Go through the level like you normally would. After you cross the bridge and get to the factory where Sokolov is, if you have the sniper, snipe the enemy closest to the wall where you are at. Then, crawl over to the grass inside of the walls. If you look around the corner of the fence you should see a guard, if you don't, then just wait for him. When you see him, snipe him as well. When you snipe him the alert timer will appear. Then turn to your left and look at the stairs leading up to the second floor where there is just a little platform. There should be a guard kneeling down up there looking around. Snipe him. Now listen carefully and you should hear the "Yankee Doodle" tune being whistled by Sokolov or someone else.