Well, if he had congenital amusia, as many people seem to think, he was born that way. It's a neurological condition, involving the actual physical wiring of the brain. It's been found to have a heritability of 70-80 percent. So it would be interesting to know if other family members had the same condition.
One Grant biographer (sorry, I can't remember which one) did suggest that perhaps one reason Grant's very pious Methodist mother did not, surprisingly, make him go to church with the family when he was a boy is that church services involved a lot of music.
I hadn't heard about the church thing. Very interesting, especially for the times.
Your post caused some odd neurons to fire such that I now wonder what a lullaby sung by, and for, an amusiac would sound like.