Unsynced audio upon exporting?

So, I was trying to make the annual end of the year video for my mom's kindergarten class that I do where I record their program and then burn a dvd of the clips for the kids to watch themselves. This is my first year using Hitfilm for this and unfortunately, when I export the file, the audio slowly slips apart from the video. The first half is mostly fine, but the longer I watch the video the worse it gets until the end when it's several seconds apart. I use the Panasonic GH-2 and used the onboard microphone not an external mic. Does anybody know how to fix this? I'm really disappointed that I wasn't able to make the video for my mom's class and I'm also concerned about my future movies having this same issue. Everything is fine in the timeline, it's just once it's exported that it has trouble.