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)))))))) (. .)))
Some voices could be heard, but their identity is unknown.
( ( ( ( ( ( Some voices ) ( could ( be heard ) ) ) , ) but ) ( ( their identity ) ( is unknown ) ) ) . )
(ROOT (S (S (NP (DT Some) (NNS voices)) (VP (MD could) (VP (VB be) (VP (VBN heard))))) (, ,) (CC but) (S (NP (PRP$ their) (NN identity)) (VP (VBZ is) (ADJP (JJ unknown)))) (. .)))