Digital Camera Repair Questions? Ask a Camera Expert.
Unfortunately the sounds quality issues are most likely where insulation has been used to reduce noise from the camera mechanisms such as zooming, there is a thread which discusses this at length here.
In most cases I would suggest an external mic, but there isnt an option on your model, in terms of sensitivity of the inbuilt mic this isnt something you can change in the preferences. If you wanted to increase the volume of sound, you could do this in the production stage editing the sounds in a video editor on your computer.
I appreciate this wasnt the answer you were looking for, but it does appear to be a known defect with this model.