Have one group of students record a video without sound (just miming, there will always be some background noise). Then upload that video to brainshark. Next, have a different group of students watch the video and record narration. Present the videos with the narration and discuss the difference between the final product and the miming group’s original ideas.
Here is an example video I made above. I recorded myself miming some stuff. I used a flip cam, but you can even use the video capabilities on your digital camera or cell phone. Afterwards, I recorded a narration over it and voila!