I'm finding that the 'focus' of the media keys seems inconsistent.
I was using Clemtine to play podcasts and could pause directly from the play/puase button.
A recent reset had me opening up tabs and one of them was a soundcloud page that auto-played. I paused it via the mouse.
I loaded up Clementine and found that instead of starting and stopping with the media key, instead it started and stopped the Soundcloud audio from the Chrome browser.
I closed the browser window, but still no love.
I've had this happen before, and simply stopped using the media keys for a while. Then they came back. They obviously work, but how can I control what audio app they're 'focused' on?
I discovered the answer through googling 'Ubuntu Media Keys Focus'.
Once Chrome takes media key focus, it keeps it, but you can turn off it's control of media within it's deeper settings.
Use this, and disable the media key handling. Restart Chrome.
chrome://flags/#hardware-media-key-handling
I learned it from here: https://www.omgchrome.com/chrome-google-music-media-keys/