Having problems getting your media keys working with Google Music in Chrome? The first thing to check is in Settings » Extensions and then scroll to the bottom of the page, then click on Keyboard Shortcuts.
If the media keys are already enabled, but not working, there is one other thing to check. If you are running Windows, uninstalling the Microsoft Mouse and Keyboard Center software should get it working. It may not be an option for you if you need some feature that it supports for your mouse or keyboard, but it worked for me and I don't really need it.