In a dream, there cannot be only the manas without the mind consciousness; otherwise, one would not even know one was dreaming. The knowing that knows there is a dream is the mind consciousness; that which can think and can be known is the mind consciousness. The deliberative nature (manasikāra) of the manas generally cannot be observed by ordinary people's mind consciousness.
Realizing the manas is relatively easy, but observing the operation of the manas is very difficult—even more difficult than observing the tathāgatagarbha. The mind consciousness and the manas are twisted together, and they are extremely hard to separate. Countless people can only observe the mind consciousness but cannot observe the manas. When the dharma concerning the manas is taught, these people say it is about the mind consciousness—their wisdom is far too lacking.
Why is it that when one is about to fall asleep or about to wake up, it is easier to discover the manas, easier to permeate and influence the manas, easier to be in accord with the manas, and easier to observe the manas? Because at these times, the discriminating nature of the mind consciousness is weak and rarely active, so it does not obstruct the manas.
When one is asleep in the middle of the night, playing a recording also works, because the manas does not sleep. The tathāgatagarbha, through the ear faculty, continuously transmits the objects of sound into the supreme faculty (indriya), and the manas can know all of it; it is just that the discrimination is not very clear. If the sound is too loud, the mind consciousness will wake up to discriminate the sounds.
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