Sentence:  He put his cigar in his mouth, and, with his right hand, up in the <WSD>treble</WSD> keys, he began to play, in octaves, the melody of a song called "The Kinkajou," which, somewhat notably, had shifted into and ostensibly out of popularity before he was born. 
Meaning of the word marked with <WSD> tags:  Pertaining to the highest singing voice or part in harmonized music; soprano.