It is currently based on ResponsiveVoice, an HTML5-based Text-To-Speech library designed to add voice features to web sites across multiple platforms.
ResponsiveVoice supports 51 languages through 168 voices and is free for non-commercial use.
Current release (patched) is installed in the Brazilian-Portuguese version of the Olympics site, where you can find it enabled on news articles.
We have identified some issues that need to be fixed and we expect to have a new release in the following days.
If you are interested in this feature, let us know.
Share and enjoy!
PS: bonus points: it should work under Plone 5… test it!