You need to carefully read the terms and conditions of the media visa that you have in order to answer your question. Most media visas that are issued to a person specifically state with whom that person is allowed to work for and no one else.If the "freelance" gig that you are writing about is not specifically covered under your media visa, then most likely you are not allowed to do so. I would contact an immigration attorney to confirm the terms and conditions of your visa that you presently have with respect to whom you can and cannot work for.