Why can we know what is being said in a dream without hearing the sound of speech?

The production of vocal sounds requires certain conditions. First, the mind must have initial application and sustained examination (vitarka-vicāra) and wish to express something; then the wind element touches the navel, and the breath, following the navel, travels all the way up, touching the throat and the tongue, and vocal sounds come out. The loudness of the voice is related to the strength of the breath; the strength of the breath is related to the intensity of initial application and sustained examination, to the degree of emotion in the manas and the mental consciousness accompanying the five sense consciousnesses (pañcavijñāna-sahagata-manovijñāna), and to the mental factors (caittas). In a dream, there are only the eighth consciousness, the manas, and the solitary mental consciousness; the mental consciousness accompanying the five sense consciousnesses is absent, and the body faculty does not function much either. Although the manas and the mind consciousness have the wish to express themselves, language and vocal sounds cannot be produced. Yet even without language and vocal sounds, one can still understand what the other party is expressing, because the manas does not need language or sound to comprehend the thoughts of the other party's manas. Since the mind consciousness in a dream is not constrained by a physical body, it possesses minor supernatural powers; it can also understand the other party's meaning, and can likewise ascend to the heavens and descend into the earth.