If you don't have an audio input jack (mic-in or line-in) then you don't have much choice. But in general I think recording audio and video to 2 separate devices and syncing them afterwards will get old fast. An M10 is $200, and using the built-in mics probably won't sound great, so after that you may end up spending another $200 for mics and a battery box. Then you have to go through the tedious process of syncing audio and video.
If I were in your shoes, and I wanted to do this on a regular basis, I don't think I would go that route. For the same money (or less) I would think about trading up to a different video camera that had an input jack, along with a stereo mic that will handle the sound levels. I can't recommend any particular models, but I think probably they exist.
The majority of people on this board are audio tapers. There are probably a few video tapers, but there are in the minority. You may want to check some video-taper forums and see what they recommend for concert level audio.