Redhat sells its "enterprise" version of gnu gpl linux for a lot of $$. As long as yuou provide the modified source to the buyer, you can sell your product, but if you don't re-distribute your modifications and try to sell it as a closed source product, then there's a problem. However, its likely not to sell that well, because most musicians who are interested in linux are usually sufficiently technically saavy to do their own install.