We cannot identify whose voices can be heard.
( We ( ( ( can not ) ( identify ( ( whose voices ) ( can ( be heard ) ) ) ) ) . ) )
(ROOT (S (NP (PRP We)) (VP (MD can) (RB not) (VP (VB identify) (SBAR (WHNP (WP$ whose) (NNS voices)) (S (VP (MD can) (VP (VB be) (VP (VBN heard)))))))) (. .)))
Three different voices can be heard.
( ( Three ( different voices ) ) ( ( can ( be heard ) ) . ) )
(ROOT (S (NP (CD Three) (JJ different) (NNS voices)) (VP (MD can) (VP (VB be) (VP (VBN heard)))) (. .)))