I have ALWAYS simply used the built in mic that is in the camera.  You get the entire room AMBIENCE that I much prefer to going through the sound board especially if the ORCHESTRA isn't miked as well and it can be controlled for video recording while also maintaining proper balances for the show itself.  I always liked all the coughing, clearing throats, paper rustling and such that comes with doing it that way because it's real and none of the shows I ever video taped EVER sounded anything less than fine.  If you have a large orchestra and WANT to have external mikes I'd suggest a couple suspended from the ceiling somehow just in front of the orchestra/stage and such so that you not only get good sound from the musicians but also the actors onstage.  I have as yet to NOT get a good recording.  SOME orchestras have been a little loud but for the most part, the better ones directed by a conscious orch director have always been kept at perfect recording levels.  And those that did not, still were OK, completely understandable, clear and balanced for the most part.  I've only seen ONE school I worked with whose sound guy knew what he was doing - actually HE graduated from that school and went on to build his own sound and light company so he always came back and did the sound and light tech for his alma mater.  And HE knew how to keep not only the balance for the mikes on stage but also kept recording levels on both the orchestra and the people onstage.  GREAT balance ended up sounding like a studio recording complete with enough audience and room ambience that it added to that "live concert" feel but better than that.