not sure if you'll be getting notifications on this, but this study (or one like it) exists, although i can't find it myself either. the one i read about didn't use voice recordings, though, just two people of different races raised in the same english-speaking locale lecturing the same class, and the students being asked to rate the lecturers on their english accents, where the students invariably marked the non-white lecturer as having a heavy accent :\