You should have a 30-60 second section of any song (regardless of it is from the show) and side from the list provided on the description of the sign-up form.

Sides are a small section of the script actors are given before an audition.

No, all auditions will be "a cappella", meaning they will be sung without any music or instrumental behind them.

A callback is a second round of audtions.

If you are invited to join the cast but don’t get a callback, you will just join the cast on the first day of rehearsal

The ensemble is a group of cast members who are assigned a variety of supporting roles in the cast. The director will assign ensemble roles as rehearsals start and they see what is the best role for the ensemble members. Many times the ensemble will have multiple roles.