The VoiceThread tool in Sakai allows for text, audio and video commenting on uploaded multimedia. You can use VoiceThread to create a discussion that is linked to a multimedia item such as a PowerPoint, short video or song. Students can comment on the item and talk to each other by voice, video or text. Students can create their own VoiceThreads as well. 

VoiceThread Resources
How to create a VoiceThread
Adding the VoiceThread tool to your Sakai course
Commenting on videos using Voicethread
Teaching with VoiceThread in the Hindi Classroom