The sentence "You could have heard the sound if you weren't asleep" is a conditional sentence using the third conditional structure. It implies that hearing the sound was a possibility, but the condition (not being asleep) was not met. The phrase "could have heard" suggests a missed opportunity. "If you weren't asleep" suggests that the person was actually asleep at the time. Thus, the sentence conveys that the person was asleep and, as a result, did not hear the sound.