You shutdown the TextToSpeech in onPause, thus mTts is not binded to the Text to speech engine anymore. You need to move your code in onPause() to onStop() if what you do is to only to show the recognizer dialog.
When the speech recognizer dialog is shown, your activity onPause() is called, but not onStop() unless your activity is not visible anymore. You should also instanstiate the text to speech again in onStart().