Here are a few ideas that might work.
SlideRocket
According to their description in the Google Apps Marketplace, they have "reinvented presentations with its feature rich, innovative platform that lets you create, manage, share and measure your presentations in one complete and integrated application". This includes audio features. I have not added this to our domain yet, but plan to and play with it soon
Slidecasting
Another possibility is Slidecasting with Slideshare. Here is a brief tutorial.
You would have to export the slide show from Google Presentation, and then upload it to Slideshare.
VoiceThread
Voice Thread would work. What I would suggest is downloading the Google Presentation as a PPT file, then uploading it to Voice Thread.
When you upload a Powerpoint to Voice Thread each slide appears as a separate slide and you can then 'record" a comment on each slide.
Here is a great tutorial
http://at.simmons.edu/blendedlearning/learnhow/contingency/guides/VoicethreadCreate_faculty.pdf
Screenr, Jing, or other Screen capture solutions could also be used.
Simply play your Google Presentation and narrate while you are recording it using your favorite Screen Capture solution. Many of these also offer hosting of the completed movie.
Let me know which ones you try and any tips you come up with in sharing these.



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