Hi Ben,
I don't see this verse in particular meaning that 'God' ONLY speaks to his prophets in dreams and visions. That is one way he can communicate, but expressions in the scripture reveal God communicating in other ways as well, thru actual visitations (you can debate the nature of these 'visitations', 'divine manifestations', 'voices', etc.). You'll have to prove in every instance 'God' only spoke thru 'dreams and visions' to individuals thru-out history, exclusively.
Also, since you only accept the Torah and the prophets (as far as I know) as the only authorative source or religious book having the truth about 'God', you're limited to only that 'interpretation' or 'tradition'. I glean from other and diverse religious texts, traditions and cultures, so may come at in from a multi-dimensional perspective.
The Deity here is speaking of
not having a visible 'form', not not having a 'personality'
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note: this Deity SPOKE apparently with an intelligible VOICE. An intelligence must have personality to communicate personally to persons. Now we can debate over what 'personality' infers or contains, but this Deity is an intelligent entity communicating to persons, they being able to understand what is being communicated. On this 'relational-level'....there is 'personal' inter-action. This 'context' allows communication, AND certainly these ancient people (whether just a mythical story or actual historical events) approach, pray to, communicate with 'God' as a 'personal' God, even if as a transcendental 'super-personal' BEING.
Again, I see different means and manners that 'God' can communicate with man. - in different "forms"
Definitely. A student of truth is always open to challenge, question and research his findings, opinions, speculations and assumptions. He is like a spiritual scientist.