A voice of reason is usually a person, or a subconscious element of the mind, which prevents impulsive behavior and irrational decisions before they become life-changing and defining. In Animal Farm, Snowball is the voice of reason against Napoleon's rash decision-making and self-serving choices which ultimately become demands. As a voice of reason, Snowball does not make decisions that will only benefit himself or just the pigs; he considers all the factors surrounding the animals' circumstances and their new-found freedom, having driven Jones off the farm.
At first, the pigs work together defining Animalism and they compile the Seven Commandments because their collective voice of reason knows that there is still a need for rules and routines. So, in chapter 2, the commandments are explained and "All animals are equal" becomes the standard by which to live—it's definitely easy to understand, although the simple-minded sheep do need some help through the process and come to understand that "four legs good. Two legs bad..." However, this collective voice of reason soon becomes distorted through Napoleon's leadership style and even Snowball's voice of reason, which should ensure that Napoleon cannot take over, becomes swayed by the concept of "brain food" for the pigs. Snowball should stand firm when the question of the milk's disappearance comes up but even his voice of reason betrays him at this moment and he is convinced that it is a necessity. The reader is forewarned of Snowball's fallibility when he spells "friend" incorrectly and paints an "s" back to front whilst painting the commandments on the wall.
When Jones tries to retake the farm, it is Snowball's good judgment and voice of reason which will ensure their victory and it is he who outwits Jones with his clever strategy, for which he is duly rewarded. Snowball and Boxer have been incredibly brave and Snowball becomes "a hero, first class," in the eyes of the animals. It is significant that Napoleon is absent during the battle.
After Snowball is driven off the farm, there is much confusion and although the animals are told that they are far better off than before the Rebellion, courtesy of Squealer's well-chosen words of propaganda, they are no longer certain of anything. The biggest change is in the definition of equality and "some animals are more equal than others" is certainly true as Napoleon dominates the farm. The commandments all change subtly and gradually the pigs become more and more dominant. Benjamin, a cynical donkey, has always known that there is far more than meets the eye in the pigs' behavior but cannot motivate himself sufficiently to do or change anything. His comments are usually subtle and not intended to challenge anyone. They are merely an expression of his opinion. His chance to become the voice of reason passes him by and when he does call the animals "fools" because they believe everything the pigs tell them and tries to make them understand, he has the potential to be a voice of reason but does not take the matter further.
It is because the only real voice of reason has been driven away that the farm descends into such a state to the point where there is, by the end, no real distinction between pig and man as "they were all alike."