What should/should not be covered in a VO cover letter? How do they differ from corporate cover letters?

Answer 1

Your greeting to the client should contain important information about their project and a brief greeting and that is all that should be included in it.

If you can take care of music or post editing for a client, if you have specific fees or if you wanted to specify that you do have an isdn or source connect session available, this would be a great place to mention that.

You want to avoid overloading a client with information as they are usually trying to scan a lot of information quickly, so polite brevity is your best course of action.