(Original source -- SAT prep books, like the one you cite, usually excerpt things that are rather old. This is both because the style is difficult and, more importantly, because they are out of copyright and can be reprinted without paying royalties. Trust me, that's important; I used to write them!)
The author is Arthur Schopenhauer, a German philosopher of the 19th century. Keep in mind that this means you are reading not only something that's over a hundred and fifty years old, but also something that was originally written in another language. Translations often use unusual turns of phrase in order to achieve something like the effect of the original language.
The Ten Commandments "The eleventh commandment" is thus a reference to a very important rule, something that is almost as important as the first and most essential laws given in the Old Testament of the Bible, which would have been a basic text familiar to every educated person in the author's time.
"Lay down the rule" is a variation on "lay down the law" (the second definition on that page is the better one)
Another source claims that the reference to "the most sensible and intelligent of all nations in Europe" is a reference to France under Napoleon, who said "Never interrupt your enemy when he is making a mistake," but while this seems plausible I don't have a definitive reference for it. In any event, that gets more into literary criticism. The gist of the bold sentence is that not interrupting, and avoiding distracting noise, is a very important principle that is practiced by intelligent people as though it were a holy law.