Here is a video showing how to narrate a PowerPoint presentation. Links (below the video) are included to the online versions of the PowerPoint presentation. You will notice the SlideShare version does not handle the narration, but SlideBoom does.
http://www.slideshare.net/proftrusty/narration-slideshare-10500806
http://www.slideboom.com/presentations/459578/Narration-Slideshare