Do you have certain musical phrases or patterns (including relative positions in tonal progressions) - or certain meanings/moods - that you tend to stutter on more? I think we can safely say that music is a form of expression, and really, language is that too. Whatever it is we're doing when we're communicating anything in any fashion, that's the general capacity we're using. This is of course not a singular matter... and "speech center" isn't either. There may even be insights to gain if we could get to a research setup (like what you mention) that was able to compare these forms of expression in some pertinent way. "Brain activity" seems a bit coarse in this regard - but perhaps in the future someone will come along and find the significant detail. :)