The lack of an on-board microphone could/should do that, too. :-) Pictures are a great, but a full-30fps video (with audio!) would greatly enhance our feeling of what it's like to be on the surface there.
Mars is also windy, and microphones can be made to be very sensitive. Plus, there might be some interest in hearing the dust from one of the common dust storms hitting the rover.
Not a high priority in terms of data, but potentially interesting nonetheless.
The martian atmosphere is extremely thin. Sound probably carries very poorly, so all you would realistically hear is the vibration from mechanisms on the rover itself.
