you may need some professional software, not free.
I found this; http://www.computercare.ca/forum/showthread.php?t=28414 which appears to fit the bill.
I've never heard of "Bigasoft Total Video Converter" but I am assuming there's no free version. Perhaps others here will have better ideas.
Used my flip ultra to record video of my 92 yo mom saying "hi" to her grandkids and great grandkids & can just barely hear her.
Is there some way to boost the audio before I put it on YouTube?