Utilize every skill and technique you know to show your talent. Use proper body positioning, breathing and enunciation.
Prepare a piece to sing that shows your skill level. It's better to choose an easier song and sing it well than to choose a difficult song and sing it poorly.
Perform scales or exercises for a range check. The director may provide you with an exercise or ask you to perform a basic scale.
Match the tone or pitch when asked. Directors listen for your tone throughout singing auditions, but some use a tone matching exercise to test your skill and memory.
Sight read a piece. You need to know how to read music.
Sing with a small group of choir members. You may get the chance to practice with the group before singing the piece during the audition. If not, the piece should be a well known song. Typically, you sing with three choir members to form a quartet.
Answer and ask questions. You want to know as much about the vocal group ahead of time as possible. Informal auditions often include more interviews than an open audition.