At the moment, there are a variety of ad-hoc ways of controlling music and video players remotely on X. There are various reasons for wanting to do this: adding files from the command line or file manager to the playlist of a running player, supporting physical remote controls of various sorts, controlling a player from a hotkeys daemon or panel applet, replacing a running player with a new one, and so on.
There should be a standard control protocol that all media players can be easily made to understand. The majority of players use a tape-transport-style interface along with a sequential playlist; the actions necessary to control these can be abstracted into a simple common interface. A reasonable basis for the protocol would be D-BUS. Any tool could then easily send the appropriate commands across the bus to control a running player, no matter what type it is. The player could similarly broadcast information about its current state.