Recently I have been 'playing' with giving feedback to students and other members of staff as audio files. Trying both making recordings using either handheld recorders or Audacity (http://audacity.sourceforge.net/). I was trying it as part of the Sounds Good project (http://www.jisc.ac.uk/whatwedo/programmes/usersandinnovation/soundsgood.aspx)
The first attempt looked at giving a summary assessment feedback as an audio file, alongside individual written feedback to a group of first year students. It end up being about four minutes and was definitely more detailed than it would have been if I had written it. It had an unexpected outcome. It was recorded using a handheld recorder and my first time doing it, the volume varied as my hand moved. Some of the student's thought this was amusing, but they must have listened to it to know!
Second area was giving feedback on staff portfolio for a lecturer training course. It is good for this as the level of detail expected was quite high and one member of staff's portfolio was electronic so seemed appropriate. It was quicker and for me I feel I gave more detailed feedback.
Currently I am using it to give feedback on group work again to first year student and storing it in the groups own area on NILE.
Audio feedback is not new but it was fairly new to me and I would recommend it as something to try. A good place to start is perhaps the recent A Word In Your Ear 2009 Conference (http://research.shu.ac.uk/lti/awordinyourear2009/) or the Case Study produced for the Engineering Subject Centre.