For operas or musicals the audition announcement usually specifies whether they want to hear ballads, arias, uptempo, foreign languages or other details, and they always provide the accompanist and you always provide the sheet music. For less formal groups that have been around a while, usually they want to hear you both solo with a song you perform well and with the group on a song that they frequently present. Often they'll let you know in advance.In either case, pick audition tunes that display your versatility, but stay well within your vocal range. Convert a little bit of that nervous energy into intensity, but don't let yourself get too uptight ... they really do want to hear you at your best.
~S~