I agree with BruceMol in

     I agree with BruceMol in that you will want a microphone and an external audio recorder. You can then sync the video and audio in post.  There is a 3rd party plugin that does that for you with just a few clicks of the mouse.  It is called PluralEyes, and it's a product from Red Giant.

     Additionally, if you have a person that can operate a boom pole, you might consider the Rode NTG-2 Condensor Shotgun Microphone Kit.  This way, you can get the microphone closer to your subject.

     Good luck.