Just in general, I would think that, the more kinds of stimulation that are applied to the human brain (up to a point, of course - obviously there can be overload), the more this would expand intelligence - just due to one component of intelligence being familiarity with what is set before one. (In the same way that a certain aspect of "intelligence" can be measured by IQ tests that are biased toward a particular culture - if one is oriented to that particular culture, one is necessarily "more intelligent" in that aspect)

But, beyond that, there seems to be a patterning that occurs that should be of interest to those of us with a passion for music. I did my final paper for child development on the physiological and psychological effects of lullabies, and found that there has been quite a bit of research to suggest that orientation to such things as tone & interval through using music with babies shows up later in increased aptitude for actual aural discrimination & performance.

And those of us who work in health care can attest to the usefulness of music therapy, both physiologically & cognitively. As far as the lasting effects of music, there may even be a part of the brain that is neurologically "hard-wired" for music - there is, after all, a type of stroke after which one cannot speak, but can still sing. :-) And research indicates that, the MORE musical exposure one has, the greater the chances of this faculty being preserved.

No one can argue that ANY research should be scrutinized before acceptance, but I have to accept the positive effects just based on experience.

As a side note - one of the findings of lullaby studies was - not surprisingly - that, in general, using the soprano range serves as a STIMULUS rather than a relaxing agent. So - if you're having difficulty with that crabby baby, try your lower register for a more lulling effect. :-)