You can clear the history of recently spoken sentences in the Keyboard mode: 

  1. Tap the Load button. This displays the recently spoken sentences just above the keyboard in place of the Prediction bar. 
  2. Tap the History Clear button (trash can icon) on the right of the spoken sentences list. 

Note: Only the sentences that are spoken out are stored in the history of spoken sentences. 

Screenshot: How to clear recently spoken messages.