I've auditioned for choirs and for bits in local theatre(many years ago). First of all ask them what they want in an audition. Usually you need to have a prepared piece that you can play or sing to show them what you sound like. Make it a piece that you do well, not horrendously complicated that you can barely manage and not so easy that anybody can do it. Then often you'll need to play or sing some other stuff from sight and/or from a lead that they give you. You'll also need to let them know what kind of music you like to play and what you've played in the past. In a choir audition the director wants to find out your range as well and so would get you to sing scales or arpeggios to figure that out.