When you are taking a piss while simultaneously attempting to carry on a conversation with someone outside the general vicinity of the bathroom and the noise of your urinary stream making contact with the toilet water muffles the noise of the out-of-bathroom correspondent's dialogue.
Man taking piss's friend - "Hey Mike, get out here. This football game is turning into a real barn burner."
Man taking piss - "I'm stream deaf bro! I cant hear you!"