I really like this mockup Arcticpaco. My only recommendation is for the currently playing track at the bottom to instead be in the appbar, instead of always visible. Instead just show the blue progress bar all the time at the bottom of the app but only present the track into and controls via the appbar. This would really open up the feeling of the app making it feel more lightweight.
I know this is difficult to showcase in a mockup, but just my suggestion. Looks great though! I love the Blue Triangle background graphics. Have you thought about making a Green/White variant of this?