The Responsive Voice service allows you to connect your own Responsive Voice API Key to circumvent the rate limits of the default usage of Responsive Voice speech within the Text To Speech Action.
To grab your Responsive Voice API key, first visit the following link and create an account
https://app.responsivevoice.org/
Once you login to the Dashboard, scroll down to where it says "Your site code".
The API key is the digits after it says ?key=
For example, the link below, my api key 1bfHsAIu
<script src="https://code.responsivevoice.org/responsivevoice.js?key=1bfHsAIu"></script>
Copy the code and place it in Mix it Up, then press Connect.