Sounds like you want voice synthesis (i.e. Text-To-Speech), complex but not impossible to achieve, though I doubt you'd get it in the web player as you'd have to rely on plugin/dll.
Both Mac and Win OS have native support for voice synthesis (though not state of the art so will always sound a bit odd/robotic), but you'll have to find a plugin that you could interface with the API from within Unity.
However unless you can find someone who has already done this, implementing it will be hard unless you are an accomplished programmer.
Whilst on an unrelated search I came across this thread on Unity forums, that would suggest that someone has already written a plugin/dll to do text-to-speech via windows native feature. Might be worth a look.