Just personally, I would be concerned that my audio comments wouldn't sound as polished as my written comments. If I'm marking up a student's essay, or providing a few comments at the end, I am very careful about what I write. I'm not sure that I could be that disciplined speaking, and would probably end up rerecording over and over again. (I'm thinking about the process of recording my outgoing voicemail message.) So, I'm not convinced that audio comments would be any less time consuming on my end.

The other downfall of audio is that you can't "scan" the way you can with writing. So, as a receiver of feedback (who admittedly is older and a student of an earlier time), I would prefer my feedback be written down so that it's easy enough to go back to without sitting through the whole thing. I know you were talking about audio comments simply to complement written feedback, but I know I'd be less likely to go back and listen to the whole file again and again. So, perhaps the kind of feedback you'd leave via audio would be different than what you'd write: expanded thoughts on what's already been written down, perhaps?

Just my initial thoughts.