I submitted an audio essay and got an interview invite. One of my buddies did not submit anything for the optional essay (there is a reason why it is called optional!) and got an interview invite too. So my two cents is that they extend an invite based on overall merit of the application as opposed to whether one submitted an OPTIONAL essay or not!
BTW, a quick update on my interview: A 2nd year student and an admissions officer interviewed me, and the questions were pretty standard - why MBA, why now, why Anderson, a challenging team experience, so on and so forth, with the exception of one odd ball question from the 2nd year student. Who is one person that we both might know and that you look up to? Overall, I think I did well and will keep my fingers and toes crossed.