Sentence:  And he began, — "Poor insect! born to flutter and to die;" — falling into the second, directly Helen took the first, till he got down to such unreasonable bass that he suddenly gave a shriek in <WSD>alt</WSD> that made Olga stop her ears. 
Meaning of the word marked with <WSD> tags:  High pitch, of a voice or instrument; especially, the octave above the top line of the treble stave. (from 16th c.)