-- he told you about a duet Isaac and his sister used to play; after she died, he wouldn't leave the stage till someone played that piece. That's still the case, isn't it? He needs to leave the stage, and he won't.
Go to Johnny Ivory's and talk to Cecil again. Choose "ask about duet" twice. The second time, use the dictaphone on Cecil (not on the piano, but on the piano player) to record the music.
Now come back to Isaac and use the dictaphone on him to play the music and hear his half of the duet.