I’m a director at a community theatre. My husband plays in some of the orchestras. The best thing to do is scan the newspaper for articles about auditions for musicals. The article will always list the director’s name and usually an email address or a phone number. Contact the director and let him/her know that you are interested in playing for the show. The director will give your name to the music director when they are ready to start putting together an orchestra. Be advised that most musicians picked for orchestras have a lot of musical experience and can sight read just about anything. They only get together for 2 or 3 rehearsals by themselves before they start playing the show for the cast. Then, they only do that maybe 3-4 times before the show opens – so there’s not a lot of time to “learn” the music.