Here is a pretty good 8 minute video on using audio feedback for student assignments. It shows how you can use Audacity to record mp3 audio files and return them to students. If you are using Blackboard you can return them as a file attached to their graded assignment – just use the paperclip icon in the feedback box.
This video shows not only how one instructor creates audio feedback but the reactions of a student.
Audacity is not the only tool you can use to create audio – but it is easy to use on both Mac and Windows platforms. Find audacity at http://www.audacityteam.org/