There are tools that can improve certain aspects of an audio recording, but this greatly depends on what the problem is. You could try audio restoration software like:
http://www.izotope.com/products/audio/rx/The way to do this is to drop your video on a video editing timeline and 'split' the audio from the video using the 'split' function. Then simply extract the audio and work on it -- but keep in mind that you can't CUT anything out of the audio completely because this will cause the audio to go out of sync when you replace it on the timeline.
For example you can remove clicks and pops without cutting anything completely out. You can say, remove a hum by dropping out a certain frequency around 100hz using a graphic equaliser. You can normalise the audio (bring the level up). All these things must be done seperately and you have to have a bit of an interest in audio to achieve the results. There is no magic bullet to fix audio problems.
