I have a paper to write for an Economy class (Pure Compitition, Pure Monopoly, Monpolistic Compitition and Oligopoly). Basically need an article that supply and demand and shows what happens to profit and make graphs, and the teacher said to choose a topic I like. So, it would be awesome if there's some article showing JL, SI, Kicker or whatever showing more profit because of an introduction of a new sub, or anything audio related.

So, what's a good site that shows audio sales?

Google would be your best bet. I'm not sure that you'd be able to find market share or anything of that sort. As far as gross sales and balance sheets, they should be publicly available. I think most oll of the companies that you mentioned are publicly traded so they have to release that info.