Comment 4 for bug 1158814

One thing you can try, which solved my issue,

If you are a chrome user, by default it hijacks your media keys even when not in focus, for Google Play Music.

You can set it to work when Chrome is focused only or disable those keys, like this:
- Chrome App Menu \ More Tools \ Extensions
- At the complete end of the list, hit "Keyboard Shortcuts"
- Look at "Google Play Music" section
- Delete keys [x] or change the type from "global" to "in chrome"

Thanks for this article: http://www.omgchrome.com/chrome-google-music-media-keys/