someone who can't stand listening to crappy music, or commercials, or whatever happens to piss them off.
therefore, whether covertly or overtly, this individual exerts control over the radio/source of music, regardless of location (i.e. car or your birthday party). can be quite annoying, but some are true gold mines of excellent music.
"I am a complete music nazi. Which is okay, because I am saving us all from your crappy music selection. Yes, I realize it is your boyfriend's sister's cousin's party."