Check out Voicethread as an interaction tool that promotes social presence through asynchronous dialogue. The dialogue can be based on a document, image, or video.  Fox example, on one page students can see the images and photos of everyone who commented on the image. By clicking the student's photo, students can read or listen to that student's comments. I like that I can see everyone who commented on one page.  
See a introductory video at http://voicethread.com/ 

Voicethread is a free, easy to use.