The internal mike just has too many compromises to make it usable, at least IMHO. If you want a simple camera mounted solution, get a short shotgun mike like the Rode Video Mic or the Sennheiser MKE 400
. These will run the audio directly into the camera if you want, which still doesn't fix the auto gain problem, but at least you'll get cleaner sound from the mike. The shock mounting will reduce handling noise, and it should be far enough from the lens to eliminate IS noise.
The better solution is to use an external audio recorder, of course, but that's another step up in cost and complexity. But it's much better.