To create a slidecast, you need to upload slides to SlideShare. Your audio file, however, can be hosted anywhere on the web- any server, file storage, or podcasting service. You link the slides & audio together using SlideShare’s synchronisation tool. Now every time you play the Slidecast, the audio is streamed from its location and plays with the slides.
Unlike webcasts or screencasts, slidecasts does not require complex recording or streaming technology or very large bandwidth. Instead it allows you to take existing media (slides and audio) and link them together using a free, web based interface.
(via Peter Van Dijck)