It never hurts to spend time getting the audio right in a video. Bad audio can wreck a great visual. Videomaker Magazine has a number of tutorials posted on their YouTube channel. This is an video on how you can get quality sound in your video.
Just a few quick things to add.
Part of the reason I harp on knowing the specs on camcorders is that if you are making the decision between two camcorders and one of them has an external Mic jack you want to give bonus points for that camcorder.
There are times when music can mask a bad sound. Those times are rare.
You might get away with swapping out the bad video and exchanging it with a narration of the event. Not going to help you if your are doing an interview video. However if you have slightly uneven vocals you might be able to use The Levelator to re-balance the sound.