I've got Razer hardware on my home rig running Win7 and on my work rig at the office running Win10. All media keys working great with MB on both machines. You've either got a problem with Synapse or some other application running on your computer is intercepting the inputs before they get to MB.
1) Test the media keys with another application, such as VLC media player. If they won't work with another player, you've got a Razer issue.
2) Check for any other applications running in the background that play sound or video. Close them. If that fixes the issue, one of the applications you closed is causing the problem.
If the above tests aren't conclusive. Describe how you performed the tests and we'll see what else you can try.
I should have clarified that I also have a Logitech G610 keyboard with media keys that stopped working as well, so I don't believe it is the Synapse software, I just pointed that out because it allows you to make legit media keys on the mouse.
I don't have any other software open that uses the media keys. I actually don't have any other programs to test the keys with.. let me think... Wait does Spotify...? I have that...
Okay.... this is... weird. The media keys started to work for MusicBee when I opened Spotify. When I close Spotify, they stop working. They also wouldn't control Spotify, only MusicBee. I am beyond confused now.
Also more information I left out, I have Windows 10 Pro. I am on the most recent creative update, or whatever it is called.