Certainly "Les Mis" would not sound great without the voices. Doing this show with young people (in our case, ages 10-19) is definitely not easy, due to the complexity of the music (tricky rhythms and pitches as well as the range of some of the parts). Fortunately, Long Island is a place brimming with lots of theatrical talent. Most high schools (and even middle schools) have very competent theater departments. There are many regional theaters, several of which cater to teenagers. So not only is there a large talent pool, but kids are getting great instruction and experience in their schools and in their community theaters. Happily, we had a great turnout at auditions. The callback process was quite challenging as we had several people up for each of the lead roles (I think there were about a dozen young ladies who were potential Eponines!). I selected some of the trickier passages for each character to see who could handle what, and the best candidates emerged. Many of the people who were not able to be cast as leads accepted roles as student revolutionaries, "Turning" soloists, etc.
It must be said that being an ensemble member of any musical theater production is an important role. In "Les Mis", the ensemble is vital to getting many of the thematic messages across. A strong supporting cast can make the difference between an average show and a great one. We were blessed with a dedicated and talented ensemble. We let them know right from the start how important their parts were. Jessica was particularly key in ensuring they created their own characters, even if they were "just" one of the prostitutes in "Lovely Ladies" or a beggar in "Look Down".
Therefore, not only cast selection, but cast development is so important for the success of this show!
Unfortunately, some parents and/or potential cast members felt that "just being in the ensemble" didn't justify all the hard work and time that would have to go into the show. So some people did drop out. Sadly, if only they knew how fulfilling it would have been to be a part of the show -- ask any member of our ensemble how they felt about being a part of this show, and I know every single one of them was glad to have experienced it, and grew as a singer and actor.