What appropriately is, entirely depends on where you live. In some regions you are expected to wear a suit and tie, or a long skirt or dress for the ladies. Around here (Holland/Belgium) the dress code is less strict and simply a nice shirt will do.
Most people I saw on auditions had no problem showing up in jeans and a t-shirt. I have however regulary heard or read negative comments about the poor sense of dress of these participants by players who sat on audition commitees. I personally don't like to leave a bad impression simply for having the wrong outfit.... (Nor for bad playing, but that has happened, alas
