The audience represents the group of people you wish to address. The voice refers to the way that you address them.
The type of voice that you use will vary depending on your audience. Some voice styles work better for different audiences. For example I’d use an informal humorous voice when writing for an audience of comedians. If I were to address an audience of business professionals I would use a more formal technical voice as they would be more familiar with this voice and would receive my intended message in a better way.