Cultivating-(Verb)
Definition: to develop or improve by education or training; train; refine.
Dictionary.com
The voice Helen Furr was cultivating was quite a pleasant one. The voice Georgine Skeene was cultivating was, some said, a better one. The voice Helen Furr was cultivating she cultivated and it was quite completely a pleasant enough one then, a cultivated enough one then. The voice Georgine Skeene was cultivating she did not cultivate too much. She cultivated it quite some. She cultivated and she would sometime go on cultivating it and it was not then an unpleasant one, it would not be then an unpleasant one, it would be a quite richly enough cultivated one, it would be quite richly enough to be a pleasant enough one.
The word in the texts means that they were, improving their voices.
