First thing I noticed is that the volume control is hidden in revealing toolbar. I think that this is so essential feature, that is should be visible all the time and you should be able to tell if player is muted very quickly.
Additionally, volume icon should show at least different state when player is muted. Ideally, it should react on each volume change and change it's state.
My work and hobby is user experience and usability, I wrote a little bit about Spotify player on my blog, you can check there for details: