For sake of discussion, let's simplify the model down to just 'communication', as opposed talking which can be quite broad. When communicating, it involves another person and much is going on - including forming new thoughts, possible debate points, and trusting yourself that the words representing those new thoughts will simply come out.   When singing, there isn't much communication going on. If it's a well-known song, you're simply repeating words that the other person knows about in a well defined rate of sound. Even if you get a few lyrics wrong or mispronounce them, it's not a big deal to you, since you're just singing a song. A vast majority of people are bad singers anyway, so there is little expectation or pressure that you need to be a good singers. General communication however, is something society expects you to be decent at, or at least that's a major perception of a PWS. My point is, singing is psychologically much less taxing communicating, which greatly reduces the probability of any stuttering triggers.     Regarding the recent study of lack of blood flow in the Broca's area in stutterers, I'm no neurologist, but isn't this an obvious symptom rather than a cause? Certain ticks/unnecessary motor expressions (violent bob, knee slaps, eyes shut, mouth wide open, etc.) often become gestures to force out sound due to a prior reward experience. If a PWS found long ago that a violent head bob resulted in getting a word out, it then becomes associated with success, and that motor pattern becomes more and more established as a general neural pathway for 'speaking'. Before they know it, speech now becomes a combination of speech motor movement plus an unrelated motor. Back to communication, the more this unnecessary motor movement (i.e. a head bob) is used is used to communicate, the less the Broca's area needs to be activated for sounds. The study indicates the less blood flow, the more severe the stutter. That falls very much in line with the symptom - the more other motor functions are used and relied on for communication, the less neural activity needs to be present in the Broca's area for speech.