The voice could be some imp or sprite about three inches tall, playing a practical joke. It could be a demon with darker plans. Perhaps Yahveh is lying, as he has sometimes done. All this has to be decided subjectively. Thus atheists don't believe in morality.
Social animals have morality.
Morality serves the function of maintaining peaceful and cooperative relations among group members. So, there will be some basics that are universal to all groups. These rules are justified simply by showing that they are necessary if we are to cooperate for our mutual benefit. Humans are predisposed to learn morality in early childhood, in the same way we are predisposed to learn language. Hypothetical oughts describe a causal relation between means and ends that is objectively true or false.