not an expert, just guessing 1. maybe because singing is about rehearsed phrases. 2. or maybe singing is like speaking in another accent 3. or it could be that singing engages with self-voice-monitoring mechanism in the brain in a different way. 4. or maybe the brain believes that nobody will interrupt you or mock you when you're singing. 5. or your brain is too busy with singing to focus on your speech delivery, which ironically makes you not stutter.