Sentence:  In a general way , psychiatrists were able to establish on a wide basis what many of them had always felt - that the most telling cues in psychotherapy <WSD>are</WSD> acoustic , that such things as stress and nagging are transmitted by sound alone and not necessarily by words . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)