Slidecast is a service developed by Slideshare. As Slideshare puts it "Slidecasting is a new multimedia format for viewing slide decks synchronized with an audio file". Slideshare also provides instructions and a demo on how to create a slidecast.
- Upload your presentation file to SlideShare.
- Go to Edit slideshow>Create Slidecast tab. Enter your mp3 url there
- Synchronize slides & audio using the synchronization tool and click publish