Recommendations for an external microphone
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Zacuto makes a mount for it as well (albeit not the cheapest around).
Philip Bloom has this to say about recording audio with the Canon EOS 5D Mark II: philipbloom.co.uk/2009/05/23/how-to-record-sound-w...
He also gives suggestions on how to easily synchronize various clips and sound.
That said, I imagine any pre-amp device you get is going to rock out better than the onboard mic.
It would be less of an issue on an actual scripted shoot where you'd want sound to roll separately anyway because you're going to be taking sound and shots and mixing things up a lot more. (Of course, why not just rent a good DV cam in the first place instead of dealing with the insanity of the SLR style holding, lack of auto focus and lack of another way to view the shot other than on the back of the camera?) But most of the stuff I'm shooting is unscripted moments from life (the kid's birthday party or whatnot) or doc style when it's going to just be a pain later on.
I see it kind of like getting color temperature right when you're taking photos. You can always go back in and fix it later and it won't take very long, but why not just save yourself the time and shoot it properly to begin with?