Vox populi is a Latin phrase which literally means voice of the people. In English usage, it means the opinion of the majority or what most people think.
In the United States it is most commonly pronounced (vox pop u leye), but outside the United States it is (vox pop u lee). Either is correct.
There is no plural form as this is a mass noun.
A related phrase is vox populi vox Dei which literally means the voice of the people is the voice of God. It is mostly used in a pejorative sense. A mob or riot may think that what they are doing is justified by some heavenly being.
The phrase only capitalizes Dei and does not require a comma. However, the phrase is used so rarely we were unable to find even one reference in English that spelled it correctly. Also, more than half the time the translation was given directly after its use.
Because the phrase is in the original Latin, it is used throughout the world in English and in other languages. The phrase is much more common in these other languages than in English.