Define audience and voice. How do they affect each other?
The style of voice someone uses is changed depending on their audience and depending on an author's voice, a different audience will be more likely to read their books.
Audience is, like the name suggests, who the author is attempting to reach with his/her writing. Depending on the audience, an author will change his/her tone, writing style, language, and even content to target a specific demographic. For example, The Concept of Law by H. L. A. Hart is intended for people interested in Law and Social Justice and thus has a formal writing style and uses language that is associated and/or found in this area. On the other hand, The Giving Tree by Shel Silverstein is written for a younger audience and uses shorter and less complex sentences.
An author's voice is the style of writing they use. This includes tone, vocabulary, grammar and other literary elements. Typically, an author's voice is consistent and you can therefore usually identify whether the same author wrote a selection of books by reading through and finding the voice the author uses.
Both of these elements impact each other, the author will change his/her voice to suit the audience he/she would like to reach by using more complex language, different punctuation, a more gloomy or more cheerful tone, and other aspects of writing. However, it also works the other way around; if the author uses a more serious or a more casual voice, the people (aka the audience) that will choose to read their books changes as well.