It was in a recent forcecast that was linked here not long ago I believe. I'll see if I can track it down again but the gist of it is explained in this excerpt from his site:
Before Revenge of the Sith commenced, Matthew was to supervise the re-mastering of the Star Wars Special Edition Trilogy to DVD 2004. Bringing these films into the modern soundstage of DVD and Dolby EX, Matthew and his crew spent months meticulously reconstructing the soundtracks from their original elements. An absolute labor of love, Matthew got to relive his childhood fantasy and also work his sound expertise on his favorite films.