Is it possible to control played songs from an app?

Some users of my Running App wanted to directly control played songs within my app for faster access (next, previous, start, stop).

I tried a little bit to solve that but I never have access to any songs though I have when using the build-in service (Amazon is my music provider and playing songs works fine).

Here roughly the code I used:

var availableSongs = Media.getContentRefIter({:contentType => Media.CONTENT_TYPE_AUDIO});

availableSongs is always null

Any ideas?