[Solo0815]

If you play music in fullscreen, there is no way to get out of fullscreen with out stopping music.

There's also no way of getting out of anything which is played in full screen . For example you also don't have that button for fullscreen-video. So generally, this is more a feature-request rather than a bug report as "Android-back" does exactly what you want to do.

Not saying that this is completely useless. Just saying that this option already exists and we probably don't need to care about adding an additional button to the screen for a function which already exists.

Hmmm ...There is a "back" button on every screen (window) in the upper left corner if you enable "Touch-mode". In your argumentation you don't need a back-button _at_all_ because of the adroid-back button.
Call it a FR or Bugreport,  but IMHO there should be a back-button or "close fullscreen" button in the fullscreenplayback-window. There is even a "enter-fullscreen" button, so why no "close fullscreen" button.