Sentence:  For Rachel , conceded to be the prettiest of the Szold girls - and she did make a pretty picture sitting in the grape-arbor strumming her guitar and <WSD>singing</WSD> in her silvery tones - there was no particular March counterpart ; but both groups were so closely knit that despite individual differences the family life in both cases was remarkably similar in atmosphere if not entirely in content - the one being definitely Jewish and the other vaguely Christian . 
Meaning of the word marked with <WSD> tags:  produce tones with the voice