I had a similar issue, but mine was due the Plex Google Chrome extension that it seems Plex's website added. It completely took control of my media keys... what a pain in the ***. Hope that helps someone else. My trick to see what was going on was to view the list of extensions, then at the very bottom of the screen there was a "view hotkeys" clickable, doing so showed me all the various hotkey settings, and sure enough Plex was there taking over my media keys. Hope this helps someone else!