What is a Good A Capella Audition song?

I'm auditioning for the part of Dorthy in "The Wizard of Oz" and need an audition song. I have a wide range, but prefer Mezzo. Auditions are not until August, so I have time to learn a new song. Any suggestions? I need something that will sound good when sung A Capella (all auditions have to be A Capella) and will fit the musical.