Bad sound: Any seasoned filmmaker will tell you that having good sound is more important than having good images. So, if you're in the market for a new, fancy camera, you might want to consider funneling some of that investment into audio instead.
Not enough sound effects: This is a classic mistake. Sound effects help convince your audience that what they're seeing is real (not a staged exercise in filmmaking) and add depth to what should be the lush soundscape of your project. Add rich, appropriate, and high-quality sound effects to your film.