Oh, yes, I heard the voices, but I did not hear what they said. A faint spot of colour came into her cheek. 
I heard the voices but couldn't understand them because they were whispering.
mnli hypothesis: I heard the voices but couldn't understand them because they were whispering. premise: Oh, yes, I heard the voices, but I did not hear what they said. A faint spot of colour came into her cheek. 