If you're auditioning for Eponine (who is a mezzo), you'll want a good belt song, as she has to belt pretty strongly. You could look at "Come To Your Senses" from Tick Tick Boom!, or perhaps "I Don't Know How To Love Him" from Jesus Christ Superstar. Alternatively, look at one of Kim's songs from Miss Saigon - Lea Salonga, the original Kim, also played Eponine so the vocal range is right. For Cosette (the one true soprano part in the show), you'll need a very strong legit soprano song - and you need to be able to hit a top C and make it sound effortless! I'm usually wary of suggesting Sondheim songs for non-Sondheim auditions, but look at "Green Finch and Linnet Bird" from Sweeney Todd - it's a good audition song for Cosette.