Since the last update, the in-game music player has become completely unaccesible for me (the music plays fine in the background but, no matter how much I click the music icon, nothing appears and the pause/play controls are completely missing). Is this happening for somebody else? Has somebody found how to solve it?

I can confirm the music player works correct on Gog yes [see image]

lets cover the basics;
the current build is 3.2.2 and you must start a new game but you don't actually need the new water Dlc which is to be blunt, not worth the $

look at my image, the red music icon is the most basic display... if you click that it opens up a little bigger and if you then click the yellow icon it opens fully

if all the above is true for you but you still need help then please include some screengrabs so we can see what it looks like at your end