I have sensitive ears, being a musician, so people singing off key and acting like they're the best singers around really tick me off.
Speaking of sensitive ears, I have another in a long list: People who whistle that loud, obnoxious high-pitch, glass-shattering whistle indoors. Even worse if they do it like they're pulsing...oh, man, that is horribly painful. I go to this restaurant on Wednesdays where the bartender does that every song someone sings (it's a karaoke bar on Wednesdays).