I'm a developer at Khan Academy (currently working on the upcoming Android app), and it's cool to see that there's real interest in that kind of use case! In our hackathon a few weeks ago I wrote an audio-only Khan Academy app that lets you listen to any video or the text-to-speech of any article and is controlled completely through voice commands. Unfortunately it's not quite at a point where other people can use it, but maybe I'll take the time to actually productionize it. :-)

Here's the code if you're curious:

https://github.com/alangpierce/Audicademy

And there's a feature-incomplete web version that lets you navigate to any video and play it (requires a recent version of Chrome):

https://audicademy.appspot.com