It is the discipline and the will. Playing violin is a good bit more difficult than humming a song. It requires discipline, practice, and initiative.
Intelligence is by birth, it is the hardware. Smart is how you use that intelligence. I have seen a lot of intelligent people doing dumb things, but smart one make good use of the talent they are blessed with.
So, it may not be the music per-se, but certain the attribute gained by practicing the music appears to help. Perhaps the so called Mozart effect is real, perhaps not. You have one chance of growing up. You can't turn back. Would you want your best shot for your future or do you want your future shot?
Since "a long-dead Austrian composer" isn't good enough for some, try an Italian.
If giving up Angry Bird but instead yet another round of Paganini Violin Concerto #1, are you driven to do it? Those who are driven enough to do it again and again to improve and improve, in all probability, are more driven to success.
Luck tends to favor the well prepared.
Scientific Americans, "Fact or Fiction?: Babies Exposed to Classical Music End Up Smarter" September 2007
Rather than passively listening to music, Rauscher advocates putting an instrument into the hands of a youngster to raise intelligence. She cites a 1997 University of California, Los Angeles, study that found, among 25,000 students, those who had spent time involved in a musical pursuit tested higher on SATs and reading proficiency exams than those with no instruction in music.
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Chabris says the real danger isn't in this questionable marketing, but in parents shirking roles they are evolutionarily meant to serve. "It takes away from other kinds of interaction that might be beneficial for children," such as playing with them and keeping them engaged via social activity. That is the key to a truly intelligent child, not the symphonies of a long-dead Austrian composer.