Some random thoughts from graduate school in cognitive psych:

1)Rhythm often helps people with speech production problems, a subset of motor processing. Stutterers are a good case in point, but even in aphasia, if you have a productive "block" disorder (Broca's aphasia, productive aphasia, nonfluent aphasia, etc) swinging your arms or stomping a foot will get you past it, often. The reasoning is that the rhythmic aspects of motor processes are handled through different pathways, more cerebellar than cerebral and more right-brain than left-, than the motor process itself, whether rhythmical or not.

2)Melodies, poetry, and the combination thereof we call lyrics seem to have more right-sided representations that the meanings of the words embedded in those lyrics, which are generally "kept" on the left. Thus aphasics with left-brain damage, who might not be able to speak a sentence, can often still sing songs. In fact, the more songs and poems you know, the more resistant you can make yourself to possible speech problems from left-side lesions, strokes and so on. That is because if you want to say something and can't generate a new sentence for it, if there is a previously-memorized lyric or poem containing the concept you need, you might be able to get to THAT and still communicate.