Another audience is crucial to consider as you compose your book proposal—the editor(s), agent(s), and marketing staff who may read the proposal.
You may be so fortunate as to actually know the person or people for whom you are writing. If so, your task will be easier.
If you are writing for an audience you don't know, it may help to try to characterize him, her, or them to help you focus your writing for this audience. But how do you do this for people you don't know?
• Use clues implicit in where they work and their job title.
You can assume that literary agents, marketing personnel, and editors are literate, probably even well read. If it's important for you to use "je ne sais quoi" in your book proposal or refer to Schrodinger's cat or mention the Spanish Armada, it's probably safe to assume that they will either know what you're talking about or make sure that they find out if it's essential to your proposal. If not, then you haven't yet found the proper home for your book.
• (Re)check the list and the backlist (if a publisher) or client list (if an agent).
You probably have done this before deciding to submit your proposal to a particular publisher or agent, but it may be worth doing again as you think about the people behind the company logo. At any rate, it will help reveal their frames of reference.
• (Re)check the write-ups in Writer’s Market, Guide to Literary Agents, or whatever source is appropriate.
Again, you may have done this before, but now focus on any quotations that are included to help you get an idea of personality—of course, this is not the most personal or revelatory prose, but you may get some hints.
And if you come to a sticky point, you can address your concern about your audience's background, and even do so subtly, for example:
Not all readers will initially be familiar with the basic outline of my hero Johann Joseph Fux's life—organist, court composer to Emperor Leopold I, and author of a famous text on counterpoint (1660–1741)—so I quickly establish this using a quotation and then proceed with the story.
which a) doesn't run the risk of leaving your audience mistakenly believing that the character is fictional rather than historical if they don't happen to be familiar with him but b) also is not phrased in such a way that your audience will find it condescending if they do know of Fux.
• If it seems appropriate, you could also try calling someone to get a better sense of "who s/he is."
If you know nothing about your audience and have no way to find out, it may help to choose someone you do know whose characteristics you would guess might be a match to your audience and write to that person. At the very least, this may help you achieve a consistent tone and approach.
In any case, just focusing on the fact that you're writing to an audience, not just "expressing yourself," is likely to help you write a better proposal.