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

In version 6.6

To delete recently typed and spoken words or phrases in Avaz (both in Keyboard and picture modes):

  1. Navigate to the Keyboard Mode.
  2. Tap on the "Favorites" button located at the bottom-left corner of the screen. 
  3. Tap on History button. This displays the list of recently spoken words and phrases (spoken in either keyboard or picture modes)
  4. Tap the "Edit" button.

  5. Tap on the word(s) or phrase(s) that you wish to remove and tap the "Delete" button to confirm the action.

  6. The selected messages will be deleted after your confirmation.  

For version below 6.6.

  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.