Not possible without an extension. There used to be one on the Marketplace, but the author has now put it off sale. Write one yourself or hire someone to write one for you, since it's definitely not possible using the built-in functionality.
Could you please help me write one, or at least show me where to start? This would really help me. Would you like to partner with me in making a medieval RPG called Narcissus? Here's the basic story:
A teenage boy's world is turned upside down by Narcissus, a mysterious villain of the shadows. Jacquelin (the boy) trains underneath Girout, a hermit of the mountains. He sends Jacquelin on various missions to thwart Narcissus's plan. In the end, however, Girout turns out to be Narcissus, and an epic battle takes place, in which Narcissus is defeated and forced to flee to an apocalyptic present.
Speech-to-text is incredibly complicated and you are not going to find someone to do it for you, especially not for free.
If you're REALLY serious, you may be able to implement an existing API (Google has one). Even still, this would require making or using some extension to allow GM:S to communicate with the API.
But honestly, I wouldn't even bother with this unless you are already a fairly experienced programmer. You need to keep your ambitious goals in check and focus on obtainable steps so that you can progress in your skills, and then one day you can tackle more complicated problems.