I'm going to theorize... Speech is communication. When you're talking to yourself, it's not communication. There's no other party listening. As such, you're not critiquing your speech. When others are listening, or even when you are leaving a voicemail, you're adding another process. An autonomous self-critique of your speech as you communicate. When some stutterers experience a Delayed Auditory Feedback ( DAF ) device for the first time, their stutter disappears. It breaks the feedback loop that is part of the trigger for the stutter.