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Google announced that they begin to experiment with text-to-speech on Knol. The text-to-speech engine is pretty high quality.
From their site: "We are experimenting with Audio Playback as an option for some knols, starting with a handful of English language featured knols. You can listen using our Flash player, or by downloading an mp3 file and using any mp3 player."
If a listen-button is shown next to the "print" and "share" button, you know that the Knol is available also as audio.
Read more here.