As stated, it's not possible for an AddOn to control an external music player (or any external program for that matter) including Windows Media Player. Your options as mentioned are:
- Run WoW in Windowed mode and control WMP with the WMP controls in your taskbar or use a media player that has a "window shade" mode (such as WinAmp)
- Run XFire or PlayXpert which have an DirectX overlay which can control a media player.
- Use a media player which has access to and uses global hotkeys
- Switch a Mac (WoW on the Macintosh has a built in iTunes control)
- Use a in-game music player AddOn (files must be in .wav or .mp3 format and must conform to the AddOn's specified file/folder structure.