The phrase is self illuminating. Ever heard the expression "silence speaks louder than words?" Similar idea.
I understand that the chimps being silent is supposed to be important, but their words are self-defeating when they say the chimps were in "almost tangible silence," which is fucking stupid. So were they silent or not? If they were not silent, then they should say so, specifically remarking about the low chirps and other sounds uncharacteristic of their normal chatter.
How does this sound: "The sun went down with almost tangible movement." What I am really saying is, "the sun went down very slowly, and almost seemed as if it was not moving at all."
You can get on me now about being a grammar nazi or whatever. And then after you insult me I can call you names. (Just to abbreviate the inevitable.)