You can't, you have to get a different camera.
The best being a camcorder.

Most digital camera makers treat video capabilities as an afterthought.
It was something they could do very cheaply.
They use cheap microphones, and rudimentary audio circuits.
The microphone is usually not placed in the best location, just a location that is convenient for the space available.

Some digital camera makers are starting to pay more attention to the video circuits.
Some have added the ability to plug in a good microphone,

Right now you are better off with a camcorder, they are designed with video as the number one ability.
But usually they treat still pictures as an afterthought, and you get poor quality still shots.

In the past a few cameras were build with good video and good still pictures.
To do that and do it well, you must have two lenses and two CCD or CMOS sensors.
The result was a large camera that cost more than people were willing to pay.