X doesn’t actually come with “built in” music content any more than it comes with built in video content – you have to bring it in first.
The music browser in X will link to assets you’ve imported via your iTunes library, but if you haven’t imported anything there – there’s nothing for X to find.
If you want sound effects, for example, = go to Apple’s website and search for sound effects, and they have a nice basic public domain (actually Apple licensed but free for public use) library of content that you can import into your computer – and after doing so – these files will show up in X.
If you purchase other libraries or even individual SFX files, you can do the same thing and bring them into your iTunes library – and they’ll also show up for use in X.
Hope that helps.
“Before speaking out ask yourself whether your words are true, whether they are respectful and whether they are needed in our civil discussions.”-Justice O’Connor