itunesmacos
My media keys on my MBP for iTunes 12 in Yosemite don't respond. Spotify works fine. I'm using the FN key to access the standard F keys.
These all work:
Is this a known issue or something I'm missing?
The software Beardedspice provides a user-friendly approach to prioritizing Spotify for your media keys. It runs in the background and gives you a few simple settings in the Mac menu bar. It just works.
Download and installation
It can be downloaded and installed directly from the Beardedspice homepage, or by using e.g. Homebrew:
brew install --cask beardedspice
Notes
Update 2021-01-16 - Use Beardedspice as the alternative app Mac Media Key Forwarder is no longer being updated
As there are reports of Mac Media Key Forwarder not working for some users, and the developer no longer updating the app, it is recommended to use Beardedspice.
Nevertheless, Mac Media Key Forwarder will still work fine for most people running on e.g. MacOS High Sierra, Mojave, Catalina, or Big Sur so it remains an alternative.
Download and installation
It can be downloaded and installed directly from GitHub, or by using e.g. Homebrew:
brew install --cask macmediakeyforwarder
This might do what you wanted, however I did not test it my self.
Best Answer
Do you happen to have Spotify or another media application open at the same time as iTunes? They'll hijack your media keys, passing the the controls to that application instead of iTunes.
Or perhaps Chrome is hijacking your media key keystrokes. You can get more information on that here: http://www.omgchrome.com/chrome-google-music-media-keys/