Two women. Two cultures. One music. The word sensei in Japanese literally means "one who came before," but that's not what Janet Pocorobba's teacher wanted to be called. She used her first name, Western-style. She wore a velour Beatles cap and leather jacket, and she taught foreigners, in English, the three-stringed shamisen, an instrument that fell out of tune as soon as you started to play it. Vexed by the music and...