What would scare me away from this domain as a small-scale entrepreneur is that the real problem here is legal, not technical. You can't shift your songs around because it's actually illegal. That's a hard thing to overcome with a couple of guys, some computers, and some technical wizardry. And while you can solve some of the local problems, like work out a deal with Sirius or whatever, you're going to have a hard time putting together a solid, across-the-board solution, and then even if you have it the content providers are going to try to squeeze you and make themselves very problematic to deal with. You're going to have badly siloed content. By comparison the technical problems seem almost insignificant.
Web pages we lucked out with a bit because the medium was always something that could be rendered from many types of browsers, but one of these days someone is going to say "You can't use Instapaper on this page without paying us money" and throw an epic-level legal hissy fit, and the courts may very well side with them.
Not saying this is unsolvable by any means but it's definitely a high-risk idea.