Perhaps Nintendo had some its own game engines for sale, but I think most companies just wrote their own. If you pick up a game coded by Ljn, for example, you'll notice it looks a lot different from games coded by Rare. They used totally different sound/graphic engines. It is quite possible that they used the same programs for generating usable graphics (i.e. drawing bitmaps, converting those to NES format), but maybe not for sound. And yes, there is an official NES manual, but it's just probably some crappy translation of the japanese manual.