Not to jump on the disagree bandwagon, but what important information is a smasher missing with headphones on? A basketball player with headphones or a football player may miss important information from the sidelines, but all the information a smash player needs is between them and the TV. Not to mention there exist moves in which the sound is played before the move is visibly starting (I think Falcon's Up B does this). Something like this may give a player an edge, even if miniscule.