If I select the Tools menu, then Settings, then Global Shortcuts and attempt to assign the Next Media or Previous Media keys, these are not accepted. I can only assign Shift+Next Media or Shift+Previous Media. The same for the Pause button. I can only assign the Shift+Pause key.
This is after ticking the "Use X11 shortcuts when available" checkbox. Otherwise, no shortcut key is recognized at all. Assigning them in the global Linux Mint keyboard settings does not help.