An audition is a performance for specialized audience. Get an accompanist or recorded accompaniment to work with (no vocal parts). Keep these points in mind:
Listen: Do a lot of listening, but focus more on accompaniment than some singer’s style or delivery. Think about the form of the song, how you want it to build, etc.
Tone Quality: Always round and pretty, relaxed, never forced, even at high-intensity moments. Keep the throat open and relaxed by practicing the melody on a neutral vowel (“ah”, “mah”, “lo”, etc.)
Pitch: Aim for the brighter side: think vibrations “up”, propelled by fast air.
Rhythm: Crisp and true, never mushy. Same with Consonants.
Phrasing: use the lyrics to make phrasing decisions, when to breathe/pause, and when not to. Carrying breath across phrases is an effective technique.
Expressive Aspects: Appropriate dynamic contrasts; appropriate use of vocal jazz techniques (terminal vibrato, flips, scoops, falls); natural sense of rhythm and when and how to delay a phrase for expressive impact.
Confidence: Own the song. This comes with preparation and experience. Memorize music so you can communicate/connect with the audience. Employ appropriate movement, gestures. Work in front of a full-length mirror.