She's certainly a mystery. There are about 1500 hits for "Angela Monet" on Google, each attributing to her the quotation "Those who danced were thought to be quite insane by those who could not hear the music". Yet there is no further information on her on Google (eg if she was a writer, or famous in some other field, she would almost certainly pop up somewhere on Google), and nothing else is attributed to her. Nor, despite her name, does she appear to be French - she and her quote don't appear at all on Google's French pages. I think that if Ms Monet ever existed, she is a private individual who submitted her quote to some website collecting quotations, and this has been copied onto all the other websites. (Incidentally, some websites attribute the same quote to Milton's poem Allegro (which is absurd), and to Friedrich Nietzsche which sounds more likely.