Sleep does not reflect an inactive brain and so we do hear sound while sleeping. It is a recurrent healthy state of inertia and reduced responsiveness found among higher vertebrates. It is controlled by the brain and is associated with characteristic electrical rhythms in the brain. Sleep is induced when the central core of the brainstem situated below the cerebral hemisphere is stimulated. Stimulation of the reticular formation (portion of the central nervous system which consists of small islands of gray matter separated by fine bundles of nerve fibres running in every direction) by natural sensory input, by messages from the cerebral cortex can awaken sleeping persons.
Neurons (nerve cells) in REM sleep are as active as they are in wakefulness. Mental capacity also does not decrease. Incoming sounds are subjected to ceaseless scrutiny. Unimportant sounds are ignored but important ones even if feeble cause arousal. Arousal thresholds are variable and they are a function of the meaningfulness of the stimulus.
With a stimulus having no significance, to the sleeper, thresholds can be rather high. So irrelevant stimuli are actively shut out during REM. Behaviourally, it has been established that motor responses can be evolved in all stages of sleep, but it is difficult to demonstrate that new responses can be acquired during sleep. But, in meditation man achieves a state of rest deeper than sleep indicated by slower heartbeat and breathing and so we do not hear sound.