Really neat job on this buddy Great looking GUI and a very functional layout. I love the idea of the center being transparent too. Awesome work!
3 things I can spot to suggest editing. First is the button that toggles between the bar/full interface; it doesn't have the over or down states like the other buttons. Second is to add some anti-aliasing to the text layers as the jaggies affect the smoothness of the rest of the interface. Last thing is the play/pause button; it's the only button that nudges the icon to the right on its down state. It's a good effect, but it just looks strange with none of the other buttons doing that, plus the fact that the button wouldn't move like that anyway. I would suggest making the icon smaller instead of nudging it.
But until you update or decide not to, I will just make those edits myself as I'll be using this for a while me thinks. Everything else is beautiful. Bravo
EDIT: Ignore the second point about jaggy text
Peace, Lance