We often forget that in video, the SOUND is just as important as the IMAGE. Bad audio can be annoying and distracting. Here's some ideas on how to improve your audio:
-Use appropriate audio equipment. Depending on your shooting environment, it's often helpful to use an external microphone.
-Use a sound-proof room if you are capturing audio that is independent from the video, such as a voice-over or a sound effect.
-Check out the audio help wiki for more tips on sound.