I know, I know, who cares? Well, to me the Wilhelm scream is pretty cool. Anyways the Wilhelm Scream occurs in the cutscene before the mission in China, when Rios and Salem are on the train. Rios grabs a guy and throws the man off. The man emits a Wilhelm, only to get smashed into a sign.
Confucius say: "Man who plays with fire, makes ash of himself."