JUCE is presently missing the following:
- volume up
- volume down
Some investigations have found that Linux/X11 requires including this file: https://cgit.freedesktop.org/xorg/proto/x11proto/tree/XF86keysym.h#n36
Note: it looks like macOS idiotically controls volume up/down/mute at a lower level. See: https://developer.apple.com/documentation/appkit/nsevent/1535851-function-key_unicodes
Note that some function keys are handled at a lower level and are never seen by your app. They include the Volume Up key, Volume Down key, Volume Mute key, Eject key, and Function key found on many computers.