The thick cloud is the veil of the flesh and the ordinary mind, the density through which the higher self must speak before the lower faculties can bear it. Voice comes wrapped in cloud because raw awareness would overwhelm the senses; it is stepped down into something the crowd of ordinary thoughts can hear and trust.
Moses is the drawn-out reasoning mind carried up the mountain of raised consciousness, standing between the people below and the light above. The exchange of words in both directions is the inner traffic between a person's scattered faculties and the single center that governs them, each learning to believe the other.
Cloud: the density of flesh and ordinary mind through which higher awareness is dimmed enough to be borne
Moses: the reasoning faculty drawn up out of the lower nature to meet the center
Mountain: the raised state of consciousness where a person contacts what is above the daily mind