The latest version of the app comes with a built in high quality voice called 'Tom' , please go to settings to choose it.

If however you prefer to use your devices text to speech and you are not getting a voice then this normally occurs when you have not installed a text to speech module on your device yet.
Please see this guide from Google on how to do this.

https://support.google.com/accessibility/android/answer/6006983?hl=en-GB