NO duty or tax on musical instruments. There is still a "war-time" tax on jewelry and, of all things, on air-conditioning equipment. The endangered species and the cultural treasures regulations prohibit import and export of these items, but unless the instrument is made of elephant hide ....
Stringed musical instruments, not played with a bow, became free of duty Jan. 1, 1998. (Violins were duty free but guitars, banjos etc, had a duty which was removed in stages for ten years upon the agreement with the US in 1988). I guess violins were considered to be less disruptive of national peace and security. All of these duties are gone now.
Note- visitors bringing materials across the border can be asked to declare them, and insure that they are taken back across the border when they leave.
Thread creep. My daughter imports Navajo jewelry for her old-time boutique. The jewelry tax is payable on necklaces and rings, but the popular silver and turquoise belts are free because they are classified as wearing apparel.