. . Hi, Eugenio.
. . This API is only available for extensions. You can port your logic to inside an extension (people would have to install it to use, of course), create an extension that exposes the functions to the "outside world" (people would still need to install the extension to use your app correctly) or simply use a client-side synthesizer (speak.js, for example).
. . You can use WebAudio API (or event tags) and calls to the Google Translate TTS endpoint, but that's not a Public API and it has no guarantees. It can simply stop working because of some limitation from Google, they can change the API or endpoints and yadda yadda. If it's only for testing, that'll probably do, but if it's a bigger project (or a comercial one), I strongly advise against that option.
. . Good luck.