I don't know if this works on Bway as well, but I know when an orch needs ringers, the conductor has networkings that lead him to a freelance musician who plays the instrument he needs. Then the musician is contracted to play for the concert that he is needed for.
So no, I don't think there are auditions. I think it's all on networking.
But who knows? I might be wrong. Please correct me if I am.
There is generally a music contractor who is responsible for putting the orchestra together. They select the musicians they want. Usually it is people that they have worked with before. I've never heard about auditions for the orchestras. In my opinion, the best way to get involved is to get on a sub list. They need subs for the musicians when they have to miss a show. People will get to know you, and when a position opens up they might ask you.