VOXOPOP
Voxopop enables you to start an asynchronous discussion with other people using their own voices. You just send them the link to your created discussion.
Here is an example of my discussion called Weather talk created in Voxopop.
http://www.voxopop.com/topic/701a2122-e10a-4d4a-b024-47439f60b6a8

You can sign up here and start experimenting
http://www.voxopop.com


VOICETHREAD
Voicethread is similar, also good for asynchronous discussion, yet this one allows video responses and also it automatically plays the whole sequence of speakers one after another. It is often called a group audio blog.
So enjoy my first voicethread here:
http://voicethread.com/share/1701864/
Here is another one. In this one you can see another possibility of voicethread - commenting your own photographs. And the visitors can comment the whole thing or just individual photos. If you go to the photos in the bottom right corner one by one, you will hear my comments as well as other people. You can also write your comments.
http://voicethread.com/share/1702008/

I really believe these are great tools well-worth experimenting with.
Here is a very nice example of possible class usage of this tool.
http://voicethread.com/share/327941/

You can sign up here and experiment with the tool
http://voicethread.com/

And you can learn more about Voicethread here:
http://digitallyspeaking.pbworks.com/w/page/17791585/Voicethread#WhatisVoicethread

Different opinions on how to use Voicethread in your teaching:
http://voicethread.com/about/library/

Another tutorial
http://youtu.be/arB74zm6DPs
or here
http://youtu.be/DKSsVO5RDx4