Google is testing a new feature in “Knol” that automatically converts articles to audio files, which can be played or downloaded. “We are experimenting with Audio Playback as an option for some knols, starting with a handful of English language featured knols. If this experiment is successful, we may make Audio Playback available to more knols in additional languages, and additional features,” mentions a Knol help page. For now, the option is available for a small number of knols, like this one about multiple sclerosis.
Source:→ Google System