Its crap. Asians are good at classical music and math. But if you take all the black students you will find your this theory goes to hell. A good example of correlation not causality.
OK, but now back to reality: Who needs a study for this.
Anybody with common sense can see that diligent practice in one discipline helps achievement in another?
There are tons of black students who are good at classical music?
I think if you define rap as music, the theory would go to hell. But then, rap is hell.
Music in a technical sense is not mere rhythm or chanting obscene rhymes. Now the old time jazz and blues players DID have considerable musical talent - but they also were by and large better educated than the average today.
The close relationship between music and math goes all the way back to medieval times. The original universities taught first the Trivium - logic (to reason), grammar (the vocabulary to express your reasoning), and rhetoric (the means of arguing your reasoning). But graduates went on to the Quadrivium . . . which was math, geometry, music, and astronomy. All based on numbers and ratios.
That curriculum gave rise to all of Western music, and some of the best composers were also mathematicians. For example, the greatest composer England produced until Tallis and Byrd was John Dunstable (or Dunstaple), who went to France with the Duke of Bedford to the Hundred Years' War and jump-started the entire Burgundian School. He was also a mathematician and astronomer - his epitaph is quite impressive -
"Enclosed in this tomb is he who enclosed Heaven in his breast, John Dunstable, companion of the stars. Through his judgement, Astronomy opened the secrets of the heavens. This man was your glory, your light, your prince, O Music; and one who has spread your sweet arts throughout the world. In the year 1453, on the day before Christ's birthday, he passes to the stars. May the citizens of Heaven receive him."