What is VoiceThread?
VoiceThread is a collaborative multimedia slide tool that enables users to upload multimedia in slides where their peers can interact asynchronously. Multimedia items can include documents, presentations, videos, or images, and interactions can be in the form of audio, video, or text comments.
To access VoiceThread, add the VoiceThread tool to your site tool menu.

For instructions, see How do I add VoiceThread to a Sakai site?