Yacklearning is a site encouraging educators to use Yackpack as a teaching tool. With Yackpack, teachers can record and save audio feedback on a webpage accessible by the students.
While Yackpack isn't the first or only tool for sending audio messages, there are several reasons why teachers should consider it. First of all, the web-based interface resembles "classroom", making it easy to contact specific students by photo. Additionally, since the site saves prior audio comments, it's easy to log-on later to review student progress over the semester. Watch the video to learn more.
Audiofeedback can be a great way to cut down on the time it takes to manually grade papers. Students also appreciate the more "social" and "personal touch" to comments. ("Hope you're handy with a deep-frier" seems so much more pleasant as audio commentary).
For professors still mastering the technological hurdles of the telephone, consider Gcast. Although there isn't the same portal/community interface, you can easily dial an 800 number and have your comment sent to students as an audio file. Both of these services are free, but ad-supported. If you don't want ads in your comments, consider Skype or even the new Skypecast. It's just like a group study session, but you can do it at home in your underwear. Actually, that's kind of creepy. Don't do that.