It's not for just one frame though. It's as if either before or after the audio the default or rest position becomes open and not closed. Even in the tutorial if you follow all of the directions but give a little bit of time before the audio begins you will notice that the mouth is open from frame 0...

I have either the same or a very similar problem. Before the audio starts playing the mouth is fully open, then during the audio it works just fine, then after the audio the mouth is closed, until the clip repeats and the mouth goes back to full open. I assume you are using the volume version of the...