a link to a sample of feedback I've given students
note: I just grabbed a file at random from several years ago at another institution. I have better recording quality now and I do better job of editing and focusing my comments, but it should give you a feel for the kinds of things one can do via audio that are harder to do in written text.
My basic strategy has been to take notes while the students perform and then record thought for them as quickly as I can afterwords.
I have also had some success with audio reading guides
Here are guidelines for Dowloading and using Audacity