I use a keyboard that lacks any multimedia media keys (volume up, volume down, mute, eject).
Since macOS doesn't allow you to remap existing keys as media keys I wrote MediaKeys to allow you to do just that.
You must leave the app running in the background for the remapping to occur.
Go to MediaKeys -> Preferences to configure the key mappings.
MediaKeys uses ShortcutRecorder.
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Contributors: David Dauer, Jesper, Jamie Kirkpatrick, Andy Kim