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And here is the Khan Academy website:
The physics and math tutorials use a lot of graphics, though, so you'd probably still want to watch the videos in those subject areas.
There may be some software out there for viewers to use (wasn't Dragon Speech one such?) to produce their own transcript.
If you send them a comment requesting information about transcripts, I'm sure someone there will respond to your question. As they get further into Liberal Arts subject areas, transcripts will become even more important, I think.