Yeah, the whole Silverlight 8, WinRuntime, Win 8 thing can be a bit confusing. Just as a tip, take a look at the XNA stuff as well. I don't know if you'll be able to use it with the WinRT/Universal stuff, but there's lots of media stuff there (I built a music player back in 7.5 days....hmm, I should revisit that...eventually).
Thanks for the heads up. Now that you mention XNA, I remember messing with the MediaPlayer and the MediaLibrary classes. They were decent but I never got to get the Media Association to work. Man that brought up some bad memories, it almost felt like they made sure we couldn't go far with anything media related, or maybe that's just me.