Is commentary over movie audio transformative enough to be considered fair use?

Interesting question. There is a four-part test for fair use. The first two parts deal with the question of the purpose of the (1) original work (the movie),  and (2) the use -- your audio commentary. While a podcast can be strictly educational, most seek revenue through the sale of ads, and the like, so your use would probably be considered commercial -- as is the movie. Those two tend to cancel each other out, so now we go to the remaining two parts, which are usually the most important. (3) How much of the original is being used? If you just using snippets to illustrate a point (e.g., from The Empire Strikes Back, "Luke, I am your father!" "Nooooo . . . ."), that is a great example of fair use. However, if you are using the entire soundtrack, then that is likely to be found to be an infringing use. (4) What is the effect on the commercial value of the original work? Commentaries are now an important Blue-Ray feature, and the studios would argue that copying the entire soundtrack for commentary would likely diminish the value of their product -- and I believe that a court would likely agree with them. Thus, if you are using a few (2, 3, 5, etc.) minutes of soundtrack, that would have the best chance of being found to be fair use. Using the entire soundtrack, however, will most likely result in a lawsuit for copyright infringement, and a finding of liability.