Play/Pause/Prev/Fwd buttons not working
I found this to solve the problem for me, thanks to Major Hayden!

...check your keyboard shortcuts in Chrome:


Choose Preferences in the Chrome menu in the menu bar
Click Extensions in the left sidebar
Scroll all the way to the bottom of the page
Click Keyboard Shortcuts
Look at the key bindings in the Google Play Music section
Remove them all by by pressing the 'X' icon next to them.